I am booked at Dreams Tulum for June 11, 2011. We had to book almost a year in advance to get the date we wanted...hope you don't run into any problems in that area. I would recommend contacting the resorts you're interested and see what dates they have available in May...that might help you make a decision.
Originally we thought we would only have a few guests willing to fly to Mexico for our wedding, but so far we have 20 booked and paid for--it was quite a surprise. Congratulations and Happy planning.