What does this gdb output mean?

That is a confirmed bug of the iOS SDK 5 / Simulator - happens as soon as the simulator tries to play sound (from a movie or any other sound source). Do not worry though, it won't be a problem on the actual device.

You mentioned actual reports of users having trouble using your App - those problems are not related / connected to the issue you have posted above.

This issue is filed by Apple under Bug ID# 10555404. I did file a report myself which has finally been identified as a dupe of the mentioned bug id.

The issue currently persists up until and including Xcode Version 4.4.1 (4F1003), iOS SDK 5.1.

Update

This issue is finally fixed within Xcode Version 4.5 (4G182), iOS SDK 6.0.

Note

The issue persists even in Xcode Version 4.5 when used in conjunction with the iOS 5.1 (or lower) simulator.

I found a temporary workaround to this problem: simply turn off breakpoints before playing the video. Then it works fine in the simulator.