Quick Summary:
We have a completed flash game similar to Tetris that is ready to be \"linked up\" for multiplayer. After doing some research and asking here: Flash Sock
I have used Flash Media Server to develop a number of multi-user apps (including games). Personally I think it's too pricey for what it does for you.
I've never used Red5 or Node.js, but the socket connections from Node.js sound promising.
I've also built a simple never-ending-socket using PHP which works as well (but not as good, it's a little jumpy) I bet you could find more info on that and perhaps develop something yourselves seeing as how you won't have to transfer too much data between users. On the web there's a number of examples of using PHP sockets with *SWF*s like this chat room example on kirupa.com
Best of luck.