Arizona Governor Jan Brewer decided against causing a frontloading free-for-all in the 2012 presidential primary season by moving up before their current February 28 date (January 31). With the exception of Iowa, New Hampshire, Nevada and South Carolina, no states are to start formal procedures before March 6. But, February 28 is less a problem since South Carolina, which is scheduled on that date, could just move up a few days and not start pushing events into December.
If no other states jump the gun, and several are still considering it, the opening nominating events are set and start with the Iowa caucus on February 6. Colorado is on Super Tuesday, March 6. It is likely the Republican nomination will still be at least a two-person contest on March 6.
There is a considerable lack of coherence and responsibility to the current calendar. Others states, along with Arizona, considering going before March 6 are Florida (January 31) and Michigan (February 28). If states have official events before March 6, they lose half their delegate vote at the convention. They could stage debates or non-binding events to gain attention.
See Washington Post article: 2012 primary calendar: Arizona starts the party