I don't see anything wrong with a contest. I get SvW's thoughts on keeping the site "pure" and not ending up with a GeoCities style front page, but the idea is no different than concepts ranging from a lunch-and-learn to Interop.
People show up at a an IT event/symposium/etc. and what happens? They learn a little, interact a little, and then stick around for the drawings/contests. If I'm wrong then why the mass exodus when the last name is drawn? Why the hours spent getting stamps/sigs from various booths to get a t-shirt? Because that's what people do. Some may have risen above this or simply frown on it as polluting the sanctity of IT with consumerism/commercialism but it is still very prevalent.
So, you say "I go to Interop/etc. and don't participate in that contest crap." OK, but do you stick around for the after-parties? Or go party with other IT folks you know while there? That's what the Comms Room is to some extent here. Adding things like announcements/news via either a tab at the top or a weekly email might be worthwhile.
I haven't been here from the beginning nor do I have tribal knowledge of what the founding team was striving for...but I don't see any harm in having a contest from time to time. It helps get people involved in something, and just might be the thing that attracts the next 50k member.
All that said, since the original question was "what sort of contest, who should sponsor it, etc." I don't really have the creativity to come up with the contest idea or rules. But why not get a sponsor such as O'Reilly or an online CBT course provider to offer up a training course(s) as the sponsored prize?