Our Florida State blog, Tomahawk Nation, relays a tweet from ESPN's Bruce Feldman:

Stunned to hear #Urban Meyer resigning, altho a coach I know ran into him 2 wks ago & said he looked so gaunt he barely recognized him.

Though not confirmed at this time, it's possible that Meyer's health concerns stem from a brain cyst discovered in 1998.

Tomahawk Nation reflects on what this means for the Gators:

For the first time ever, both Florida and Florida State will have new head coaches in the same season.  Florida also lost its longtime defensive coordinator, Charlie Strong, who is now Louisville's head coach, and its best offensive assistant, Billy Gonzalez, who left to take the offensive coordinator position at LSU.  With National Signing Day coming February 5th, UF must quickly find successors to Meyer, or risk what was shaping up as the top recruiting class in the nation.  That recruiting class is especially important as the Gators will likely lose up to 12 players to the NFL, including multiple underclassmen.