Hey I also got engaged on Christmas And we too are having a Mexican wedding in January of 2011. I know you said you wanted it near cancun, but have you concidered Cabo san Lucas, In the Baja (BCS)? We stayed at the Hilton there last year for my fiance's birthday. AMAZING!!!!! The beach was beautiful. No Seaweed at all! When we started planning the wedding we looked at each other and said "CABO!" It is a true paradise. I can't express how wonderful this place is. Look up some photos or if you would like I could send you pictures.
I've never been to Cancun, but it is a pretty big tourist place. A lot of my friends have been and everytime I hear it is crowded. When we went to Cabo, I think that there was maybe 10-15 other people at the Hilton. ...and if I'm not mistaken -Dec.-March is their main tourist months. Other than that everything is pretty much calm, so if you wanted something more personal with not a lot of tourists, that would be another reason for Cabo, but that depends on your personality along with your wedding party/guests.
Just some food for thought
Congradulations on the wedding, It'll be a wonderful day, no matter where you end up. best wishes!