Her last day is January 31st. According to this AP report, the President accepted the resignation “reluctantly.”
Tony Snow’s explanation on why she’s leaving:
“Basically, she has been here six years.”
Update: Tony Snow is now talking to the White House Press Corps. Ms. Miers said that it was just time to move on. There’s no successor in place yet but she wants to stay in the position until the end of the month to make sure the transition goes smoothly.