It seems that RDP for Mac has a tendency to hang if you Apple-Quit out of the program. Since I'm relatively new to my MacBook Pro 17", I was rebooting in order to free up RDP for another session.
Tired of my own laziness, I decided to do things the right way and find out how to Force Quit an application in OS X. I'd love to take credit for such a well-written document, but the folks at Ohio State beat me to it and put together a great little page describing how to kill a process under Mac OSX. I couldn't have done it better:
Be aware that after killing RDP using Force Quit, it does take a couple of tries in order to login again to the remote server. Also, you'll experience less RDP hangs if you gracefully logout of your Windows session by using Start -> Disconnect, rather than bluntly Apple-Quit'ting out of RDP. Word to the wise..go gently into that good night.
Also, you can only run ONE RDP session on your MacBook Pro at a time, so its use is limited if you need to login to many servers. One idea to get around this limitation is from your Mac, login to one jump server that will allow you to get to all other boxes you need to support. This way, you can have your Mac Cake and eat it too!
Finally, if you've left a session open and another RDP session is started from another server going to the same remote host that your Mac previously had control over, RDP on the Mac tends to hang. Obviously, this necessitates a Force Quit again.