It's completely open.  When you vote, you get a ballot that contains all the Republican names AND all the Democratic names, with instructions that you can only pick one candidate from EITHER column.

On my drive to work in the morning, I pass no fewer than THREE homemade Ron Paul 2008 signs on the side of the roads, painted onto businesses and barns.  With Michigan not requiring Republican status to vote in the primary, and Democrats being disenfranchised, Ron Paul has a reallly big shot to win Michigan, at least more than any other state.

Hmm, MI has been known to do some pretty weird stuff.  

She didn't really.  More that she took advantage of an opportunity in which all the major dem candidates (Edwards, Obama, Richardson, etc.) took their names off the Michigan primary ballot.  Hillary implied she too was going to do this, and pledged not to campaign here, etc., but by the time the deadline rolled around, Hillary thought it'd be best to stay on, seeing the advantage of being the only major candidate on the ballot and, if Michigan got to keep its delegates, she'd pick up a lot of votes from the 8th most populous state.  :)

Behind the Michigan Primary Moves

The industrialized midwest has born the brunt of trade, trickle down, union busting, and loss of high wage jobs with good medical benefits. Yet, it has no say about any of that or who the hell is President.  Remember, it was Willie who passed NAFTA with help from the Republicans against Democrats and the unions.  For this reason, I support MI rocking the boat.  Because it will help Hillary, I'm pissed.  How's that for mixed emotions?