Random prompts for someone else's Apple ID password after upgrade to iOS 5

I just installed iOS 5 (was previously on the beta via Developer program), and I'm having an issue where sometimes I'll get prompted for someone else's Apple ID password. It's different from the typical prompt in that the title of the dialog is "Apple ID Password", the Apple ID input field is editable, and it's NOT using the saved Apple ID from settings. Entering my Apple ID/password has no effect. The first time it appeared, it was installing the apps that were previously on the iPhone right after the upgrade. None of the apps would install. So, I went into the App Store, installed them manually, and everything was fine. In general, installing apps works fine - it uses my normal account.

I thought once I got all of the apps re-installed, the prompt would disappear (thinking the bug was related to the re-installation of apps after a restore). However, just now it appeared as I was syncing music to it via iTunes. I've tried restoring multiple times from backup, but it keeps happening. In the iPhone Settings, under Store, it's my Apple ID, and I've tried signing out and signing back in, but this has no effect. The Apple ID that's popping up isn't mine, and I've never seen it before. The phone (iPhone 4) was bought new over a year ago. Does anyone know how to fix this?

Thanks!