If you're looking to run a HTPC for XBMC solely to stream your content from your NAS/Plugins then you may want to keep an eye on the development of the Android branch.
As you know there's a myriad of Android based devices from Pivos Xios DS through to the upcoming Ouya games console that can run XBMC decently (compared to how the rPi run's XBMC) that may be a cheaper option for some of you who don't need the PVR functionality. Around the $100 mark or even less...
You could have XBMC as a secondary function of the device and use them as intended or just g'head and use them solely for XBMC, foregoing their original purpose.
Word of warning however, if you're interested in the RetroPlayer branch of XBMC I believe there's ongoing issues with sound when use RetroPlayer for ROMs when running XBMC on Android, atm.
Just sayin.