Facebook just released version 3.1.4 of their iPhone app, and the most significant digit in that version number is the ".4", as in "iOS4 compatible." Which it wasn't, prior to this, despite Twitter and LinkedIn jumping on the iOS4 bandwagon almost immediately, and despite an update only a few weeks ago that omitted said support.
Anyway, it's out now (including minor fixes, like a flaw in the user interface from the last update!), so you can wall-post and YoVille to your heart's content. Unless you have an iPad — Facebook still doesn't have a native app for the big screen yet.