Update: MSNBC's report late Thursday night that Mike Gravel had exited the race was not true. According to Gravel's official campaign website:
MSNBC pundit Keith Olbermann has incorrectly declared that Sen. Gravel has dropped out of the race following the January third caucus in Iowa. This is not true, and Sen. Gravel is still an active member in this race. We are requesting that MSNBC and Keith Olbermann retract their statement, and issue an apology to the campaign for promoting blatantly false misinformation. Again, Sen. Gravel has not dissolved his campaign, and has no intentions of doing so.
Thus, there are still 6 Democratic candidates in the race.