Whoa, you're right. It's already available
It's still a little on the pricey side at 379.99. There are other next-gen boxes (Zotac HD-ID11) that are a lot cheaper. It's cool that this is fanless, but the Zotac is completely inaudible anyway.
In any case, it'll be interesting to hear some reviews.
EDIT: Comparing the specs of the two, (Zotac Zbox HD-ID11 vs XS35) here are the key differences as I see it:
1. The XS35 comes with a DVD-RW drive, 500 GB HDD, and 2 GB RAM, the Zotac has none of these things. Personally, I run XBMC Live from a USB stick, so I don't need the HDD. I watch mostly bluray rips, so the DVD drive is also useless to me.
2. The XS35 max LAN speed is 100 Mbit, the Zbox has Gigabit. I stream media files over the network from a server, so this is important to me.
3. No optical out on the XS35, whereas the Zotac has S/PDIF.
The rest are smaller differences:
5 USB ports on the Shuttle, 6 on the Zotac
4 in 1 card reader on the Shuttle, 6 in 1 on the Zotac
Zotac has a DVI instead of the VGA on the shuttle
no e-SATA or VESA mount with the Shuttle
As you can see, they have very different specs. The Zotac will definitely fit my needs better, but it's good to have different options out there for others I suppose.