The Mayans actually had three calendars: one that was essentially a solar calendar, another that was a religious calendar, and a third calendar that was used for keeping historical records. This third calendar is the only one that's relevant, because this is the one that some people are using to predict the end of the universe.
This long-count calendar isn't a calendar as much as a counting system. You know how we use a base-10 counting system? (10, 100, 1000, 10,000 etc.) The Mayans used a modifed base-20 counting system for keeping track of days - the second cycle went up to 18 rather than 20. So they tracked days in cycles of 20, 360, 7200, 144000, 2880000, etc.
We're coming to the end of one of the 144,000 day cycles. The concept of leap-years is irrelevant to this calendar system, because it's not based on solar years, simply on pure math. I've got no opinion of whether the calculated date of Dec 21, 2012 as the end of the cycle is accurate. But if it is inaccurate, it's for reasons that have nothing to do with leap years.