The trouble with XBMC supporting 3d is that the current 3DTV standard proposed relies on shutter glasses. One lens goes black while the other is transparent. Then they switch. This happens 60, 75, 90, 110 or 120 times a second (depending on manufacturer and model), so it's not noticeable... as long as the video stays synced. The glasses fall out of sync often, and different systems running XBMC will have variable frame-rates (that's unavoidable using non-standard hardware like a PC). Sony and others use an IR receiver to let the glasses know when to change based on the frame currently shown. When the glasses are out of sync, they pause and wait for the next frame before continuing. Since this happens so fast, you don't notice it. But unless XBMC can interface with that IR hardware and send signals to the glasses, or receive them and drop a frame (depending on who's 3D system you're using) the glasses/frames won't resync on their own. This creates a strobing effect, makes the picture look dimmer..brighter..dimmer, flickering like a candle, and gives most people headaches. Complicating matters, most do not simply alternate at a fixed interval, it's based on the perceived motion on the screen, screen color etc. Different glasses work at different frequencies. The proposed 3D standard only applies to how the video is encoded and how the discs are made. The sync system in the glasses is not standard, determined solely by the manufacturer of the glasses. XBMC would have to support many different standards which aren't open to independent developers. No one like reverse engineering a hundred different pieces of expensive proprietary hardware, risking patent infringement. They would need to find one manufacturer to support, who in turn would have to be willing to support them. If someone really wanted to pursue this, it would be worth talking to manufacturers first.
It's worth holding off on 3D support for now.
There are several reasons. First, there isn't really an accepted standard in Home 3D now. People seem to be getting behind Sony now, they're the top contender, but it requires licensing fees to be paid to them for both the method of converting video from 2 perspectives into something that plays on a 3D TV, and licensing on the actual blu-ray 3D disc. Their format also relies on active shutter glasses.
There are a few other technologies emerging. One using parallax projection (like you see in the movie theaters) that sends polarized light to different eyes through polarized lenses. The 3D glasses are cheap ($5.00 instead of $200+), and require no sensors or batteries, are lighter, and can be made more comfortable (like sunglasses instead of a head-strapped windshield). If you wear eye-glasses, they can even be made in your prescription so you aren't wearing one pair over the other.
Another option is a lenticular screen. Rather than being smooth, the pixels on the screen are arranged in small pyramids, with each side projecting a slightly different image. If you've ever seen the 3D baseball cards, it's similar. The perspective changes as you move around it. Tilting your head lets you see around foreground objects. The tech ins't totally there yet, but they do look awesome, and the manufacture cost is much lower. This requires no glasses.
Last, and more importantly, there are several groups using parallax (the differential between what the left and right eye see) images in current 3D movies of other formats and creating a depth map (Z-Axis interpolation). The video is then reencoded in real-time at 60fps from a combination of the two images based on your position in the room (by using head tracking). This only works for a single viewer, multiple people can't watch the same movie in 3D mode at the same time since the perspective is set based on the location of one person. The advantage is it looks better than any of the others, and requires only a cheap webcam to track your position. This is being created as a 3D codec that will be easy to implement in software packages like XBMC and requires only cheap hardware most people already have. The file size once encoded is 25% smaller than the best compression you can get on 2 video streams. Given the ease of entry and smaller file sizes, I think this is what's going to win out in file sharing and torrent sites, which quite frankly makes up most the common XBMC user's library. This is still in the early stages of development, and I should warn, the GPU usage is through the roof, about 4 times what's needed for 1080p playback. Even then it's a bit choppy on all but the optimized demos. Still, it's coming and is worthy of consideration.
In the end, I think we'll see a hybrid of 2 or more of these technologies. The trouble with glasses is that you can't choose what to focus on, things fade in and out based on the camera focus, not your eye. Minimizing the distance of cameras when filming eliminates that blur on the edges, but also reduces the illusion of depth. Combining that with head tracking and 5-6 video channels instead of 2 creates the only life-like 3D video I've ever seen. When you've demoed good 3D packages, you quickly realize that thing like "Avatar" are poorly implemented gimmicks, and not good enough for your money. With several competitors with different technologies coming to the marketplace over the next 2-3 years, I think you'll see a new format war. Early adopters will probably get a good 5-10 years before having any incentive to change hardware, but with so many new technologies on the horizon from competing parties, I wouldn't invest too much in this.
Here's a clip about head tracking from a few years ago most of you 3D buffs have probably seen already:
http://www.youtube.com/watch?v=Jd3-eiid-...r_embedded
Now incorporate that type of technology with a better head tracking device, similar to Xbox Kinnect (project Natal). Now you get the same effect in games without any awkward headgear.
Now imagine that processing power is used for real-time rendering of an animated movie. A next-gen console should have little problem with that. Non-animated movies would have to be shot from at least 5 angles, probably necessitating an new disc media, but similar effects can be achieved using the additional perspectives and Z-Axis interpolation.
Add to that parallax display from a lenticular screen and you have something incredible. This is where we're headed. There are patents being filed by Sony, MS, and several major movie studios. Once they work that out, all hell will break loose with another format war, and the end product won't resemble 3D as we know it today.
The next major breakthrough will be in gaming, since frames are generated at variable perspectives on the fly, rather than fixed to the position of a camera when filming. Keep an Eye on Microsoft's Kinnect and nVidia's 3D vision. The technologies are very compatible, and the two companies work closely on many projects. Sony will follow suit with something very comparable, based on the Current 3DTV standard they're pushing and their Playstation. You'll see the real momentum building with the next game console release from either company. Movies will catch-up shortly after.