My fiance and I are in a similar situation. We are down to two resorts in Playa del Carmen and know that we want to get married the weekend of May 7-8, 2011. BUT, we don't want to commit to one resort until we do our site visit in May of 2010. What we've settled on doing, is sending out our boarding pass STD for 'Playa del Carmen' and the weekend of our wedding. Once we officially lock down the resort, we want to send out an info brochure with more details re: accommodations and travel arrangements. Hopefully, this will give our guests time to save $ and vacation time.
I've also gotten mixed feedback from ppl but have found that there's no one way to do things so you just have to go with what feels best for you and your FI.
It's nice to know that other brides are debating over the same issues! None of my friends have had DW's so pretty sure they think I'm a little nuts ;-)