The 12 December 2007 Voter Guide published by the Democratic Party states:
"A vote for "uncommitted" is a vote to send delegates to the Democratic National Convention who are not committed or pledged to any candidate. Those delegates can vote for any candidate they choose at the Convention.
Supporters of Joe Biden, John Edwards, Barack Obama and Bill Richardson are urged to vote "uncommitted" instead of writing in their candidates' names because write-in votes for those candidates will not be counted. from The GreenPapers:
http://www.thegreenpapers.com/P08/MI-D.phtml