Unless FB are considering making it impossible for others to access it's API, building a phone seems like a better deal for Facebook than it does for it's users.
It's the twitter problem all over again; they want their data and network/platform to be ubiquitous but while reaping all the eyeball revenue from being the frontend of choice and making every other client second-class.
Android is by some metrics a good platform, but I do wonder whether it's going to be the revenue generating channel that they seem to hope it will be.