I don't expect there to be one for Halo 3 before the release of Reach. There might be one last contest for good old sake saying good bye to Halo, but I doubt it. What I do almost expect is that the staff is preparing Halo:Reach contests, which will be launched a few weeks after the release of Reach. That would bring awesome traffic to the site, and provide a great amount of maps right as Reach is released. That would seem like something they should do being it would be positive for the site, at least that's what I think.
I don't think there will be another contest in Halo 3, seeing as for a good map to be made it sometimes takes a month, or two. (However, I can make a decent map in a few days) This means that the contest winners would almost be announced after Reach came out. Agamer: good idea, I might do that right now