This is a known issue. To work around it, simply restart the OmniFocus app.
Since we have multitasking in iOS4 now, the app doesn't exit when you return to the home screen. So you'll need to double-click your home button to view OmniFocus in the app switcher. Then tap and hold the OmniFocus icon in the app switcher. When the icon begins to jiggle, tap the little red close badge on the icon. Hit the home button once more and then relaunch OmniFocus. Your keyboard should now appear properly. I believe this issue only occurs on first launch after install.
Hope that helps.
Last edited by Toadling; 2010-07-30 at 09:52 PM..