Just found out this little trick by accident...not too sure if it has been discovered yet.
When watching a video in ios 5 beta 2 the playback controls are hidden and don't pop up with a screen tap. To exit the video without closing down the app in the app drawer just pinch the screen with 2 fingers. This takes you back to the previous screen in Tweetdeck and Safari (and probably any other app.)
Just though I'd let fellow beta testers know about this since it's a pain having to fully exit out of the app each time you finish watching a video. :)
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