Well I just started to mess around with Flex 3 (beta) and the ESRI Flex API v4 (beta) and found an interesting bug.  It happens when you are trying to use bubble markers.  Here is a sample of the error message:

ESRI Flex Argument count mismatch on mx.binding::PropertyWatcher(). Expected 3, got 2. 

I know it isn’t much to go on but I wanted to keep this post brief.  For the most part Flex 3 and the ESRI API play nice, but when you run into errors like the above you need to compile your app using the Flex 2 SDK.  A real drag, but it does get you up and running.  Hopefully the ESRI folks are starting to work out the kinks with Flex 3.  Because, in all honesty, ESRI provides some of the best mapping solutions out there, and it would be a shame not to be able to build Flex 3 mash-ups.