Thanks Lan - I like the suggestion to fly into Cancun then take a bus to Chetumal. I've never been through this neck of the woods before but have read a number of posts on this and other boards about it.
Is the bus safe and worry free? How would you handle getting from the bus station to the border and through customs etc. Can I hire a taxi to help? (Once again I've not been through this area and my spanish is almost non-existent.