Actually, XBMC Live has the option to be installed with an embedded linux distro and should boot right into XBMC. I haven't played with XBMC in a while however as I haven't had the need for a thin client since I have my HTPC (running WMC) hooked up to my main TV and an Xbox 360 "extender" on my secondary.
XBMC is nice but I found it did not offer anything I couldn't do with open-source add-ons to my HTPC with WMC. Also, unless I completely missed it, XBMC will not make use of any tuners you have for the HTPC. If you are just using it for network streaming or ripped media however, it should be able to satisfy your needs.