Earlier this week I found out (and tested) that Adobe had fixed bitmapData.hitTest() in the latest RC2 version of the Flash Player. However, the stand-alone player delivered with CS5 was not updated. This means that you still have to test all your code in a browser, and not in the Flash IDE.
Now word had come from Adobe that the stand-alone player will not be updated until the final version of 10.1 is released. I was just this on the Adobe prerelease forums.
"Flash CS5 released with the beta 3 version of Player 10.1. We will
release a StandAlone player update once the final version of the Player
While this is not a huge issue, it does means that until 10.1 is finalized, you will need to download the 10.1 RC2 Flash player, and test all your code in an external browser. If not, you may still find weird bugs even though you think you have updated to 10.1 RC2.