I do not expect Urban Meyer to be fired while Tim Tebow and Percy Harvin are players at UF no matter what, simply because a school owes a coach after he's brought them a title. I don't think anyone has ever been fired within 4 years of winning a national championship at a major program. The quickest I've seen someone go from hero to zero is Larry Coker at Miami.
That said, I think if Meyer can't win an SEC championship with Tebow at the helm, it will raise some eyebrows in the Gator nation. I think Tebow will play out his eligibility at Florida, so that means Urban is there at least until the end of the 2009-2010 season. However, I think if December 2010 (year after Tebow graduates) rolls around and the Gators aren't playing in Atlanta, Urban Meyer will go the way of the 8-track. This is especially true if Zook keeps up the success he's having at Illinois and gets a Big 10 title/BCS bowl win during that time.