1. Setting up yaourt to compile as a normal user requires sudo which is another level of installation.
2. I like to keep my previously built packages for a little time incase unacceptable bugs creep into to code ( not that it happens )
To PKGBUILD you will need fakeroot (root user)
# pacman -Sy fakeroot
Next, download the xbmc-svn tarball from aur. (normal user)
# wget http://aur.archlinux.org/packages/xbmc-svn/xbmc-svn.tar.gz
# tar zxvf xbmc-svn.tar.gz
# cd xbmc-svn
# makepkg PKGBUILD
pacman -S [missingpack1] [missingpack2] etc....
Once completed you will have a shiney new xbmc-svn-xxxxx-x.pkg.tar.gz to install. note: the 'xx-x's are svn number and AUR revision.
Install it (root user)
# pacman -U xbmc-svn-xxxxx-x.pkg.tar.gz
Now you can store that package for keepsake. At a later date, you may want to create another later svn package. Simply navigate back to the location of the downloaded tarball. delete it, and start again at the download tarball stage. Dont delete your xbmc-svn dir otherwise you will have to download it all again. Subversion will download only the needed updates which is usually quite quick.
You might also wonder, can i install this package on another machine? Sure can! I have a second xbmc machine for the kids that only has the xbmc's requirements(no build deps). Couldn't the xbmc team adopt archlinux as its twiced removed brother and house an archlinux package for us please
This method takes a little more effort but I think its worth it.