HTC's upcoming "Facebook Phone" or the HTC First, will still just be an Android phone underneath. Phandroid has reported that the OS shaking along underneath the First handset is accessible by either resetting the default launcher, or disabling the Facebook Home app from its own settings.
This is quite interesting, because it could change the future of the Facebook app itself. I would expect a future iteration of the Facebook for Android app to give all Android users this ability in the near future. Most people live their entire lives through their smartphone, and most of the time it's heavy Facebook use (I'm looking at you, my dear sister-in-law) and the background OS doesn't matter to those people.
Give them their games, status updates and 'liked' pages, and you've done your job.