iPhone app nearly blank in simulator after upgrade from 3.1.3 to 4.1

Nearly pulling my hair out on this one.

I developed my iPhone app mostly using the 3.1.2 SDK and Three20. (there is no version associated with the cut of Three20 I was using, but the latest date in the CHANGES doc is May 31st, 2010.)

I upgraded to the 4.1 SDK and things mostly ran fine. I decided to upgrade Three20 to the latest version as well,开发者_如何学JAVA but I encountered the compile issue listed here (note: apple login required). After a lot of strife I was able to solve the issue with the help of that thread, but, lo and behold, the app is now completely blank.

The skeleton is there: tabbed navigation, with four tabs (the fourth is currently selected in my pic), but no data. Even switching to the previous version of Three20 no longer works. I'm baffled. Any ideas?

Are you calling [window makeKeyAndVisible] in your startup code? It was optional before but it seems in 4.0 you must call it. It needs to go towards the end of your applicationDidFinishLaunching: method.