The candidates had to sign a pledge that they wouldn’t campaign in Michigan and Florida:

The Democratic candidates have signed a pledge that would forbid them from campaigning in states such as Michigan and Florida that have sought to move their presidential primaries into January 2008.
The move ended weeks-long jockeying over which states get to hold early primaries.
Democratic leaders in Iowa, New Hampshire, South Carolina and Nevada, the four states that had been designated by the Democratic National Committee to hold early primaries, demanded in letters Friday that the candidates not participate in the early primaries of other states. The candidates either had to sign the pledge or risk annoying officials in those key states.
Sens. Joseph R. Biden Jr. (Del.) and Christopher J. Dodd (Conn.), along with New Mexico Gov. Bill Richardson, signed the pledge within hours on Friday. By yesterday, Sens. Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.), and former senator John Edwards of North Carolina, had joined them.

The early primary states are really playing hard ball. I think the next National Governors Association meeting might be a little strained.