Whilst I can see why a Facebook Phone looks really promising from FB's perspective, to me, buying a Facebook phone because I like using FB is akin to buying a Coca Cola branded and manufactured car because I like drinking soda/soft drink.
A "Facebook"-branded phone isn't such a long-shot, especially since Facebook is attempting to be your email/messaging/contacts database/... provider as well.
Likely as not Facebook will take a cue from Amazon, and produce a Android-no-Google phone (no Google maps, no GMail app, no Marketplace, basically nothing that requires a google authentication event. These will all be replaced by FB services.)
However, even WinPhone7 with deep FB integration didn't seem to really catch on with users (or at least, that wasn't a standout feature that brought users in).
Perhaps FB will stop updating their iOS/Android apps just to make Buffy look better?