i downloaded the yelp app on my iphone 4 and it immediately lost all accelerometer functionality - all apps, mail, camera, safari... i restarted the phone, shut down, and reset all settings [in settings>general>reset all settings] and still no auto rotation. certain apps that only work in LS went to LS just fine and certain pages that were in LS before the auto rotate stopped working got stuck in LS and weren't able to go back to portrait... totally weird. not a huge in convenience, but still... well, you're reading this so you get the point.
it wasn't until i read the blurb on the apple hdwr troubleshooting page
http://support.apple.com/kb/TS2802
and tried their suggested auto rotate test with the factory installed calculator app that it suddenly started working again. it took a few tries oriented vertically in all 4 orientations with a bit of a tap on the table but the accelerometer finally seemed to just get "unstuck".
it doesn't really make any sense to me - may have just been a coincidence - but it worked for me after trying everything else i could find via google searches.
a little nervous to re-install the yelp app. i believe it was the monocle feature that made my phone's accelerometer go haywire...
good luck everyone!
😀