Awesome idea! I work at CA and we'd love to support the kick *** maps made by players in some official way. We can't hold a contest (too many lawyers out there these days) and we wouldn't want to make money off of your hard work by selling them on Marketplace; so here's what we're gonna do.
We'll collect custom maps created by players, evalueate them, and then release the best maps directly to the game in the same way we release our own new maps (e.g. the ones with the "[CA]" in the front that have been downloading when you connect to LIVE). Of course we'll give credit to the map creators.
Please send me a personal message (click my gamertag and choose "send this user a message") if you have maps that you'd like considered for release - we'll get in contact and work out the details.