Where did you read that Skybus goes to Cancun and Belize City? Was that back in mid-2007 when the rumor mill was running?
Apparently they've applied to the U.S. DOT to serve Cancun, but it's not currently a scheduled Skybus destination, so far as I know.
As to Belize City, I suspect that would be way down the list of additional destinations, given the low traffic into Belize.