Congratulations! I too just got engaged (6/4/2010) and am looking at the Cancun/Riviera Maya area. I haven't picked a resort yet, but I am thinking about the Dreams brand of resorts. Their website has a lot of really detailed information about weddings, costs, and options. If you still haven't chosen a resort, I think they are worth a look. If you find any great resorts, I would love to hear about them...
Courtney