If you are "fortunate" enough to have Microsoft Office Communicator (MOC) be your default IM system you will notice that it doesn't work correctly after the IE9 Beta install.
First off, the typing is about a full second lagged behind what you've actually typed on your keyboard. This is VERY noticeable and make MOC completely unusable in my opinion. Secondly, MOC will occasionally turn blue, then freeze for a few seconds, then return back. A behavior I've never seen before installing the IE9 Beta.
Sadly uninstalling the beta only fixes the typing issue. MOC now freezes and turns solid blue about two times per hour. It fixes itself after 3-5 seconds, but is still ultra annoying!
So in short, IE9 can suck my sausage.