Cool idea. I tried to do that a few years ago: http://www.otcsw.com/jario.php
I never got around to coding the online multiplayer and the game was very buggy, but it actually turned out to be incredibly fun (New Super Mario Bros. did largely the same thing I did, except you can’t control enemies in theirs). So if you’ve got the drive to get an online Mario-esque platformer going, count me in.
At this point it seems like a very very young project. I couldn’t find anyone else to play with and the whole game seemed to move very slowly and had literally nothing to do. Also, the floor system didn’t seem to work perfectly - I was able to just mash the jump button and make it all the way to the top of the castle.