The Republican primary contest in Michigan is an open contest. This means that voters from any party can show up at their polling location and vote in the Republican primary. There is some concern that Democratic voters in Michigan will show up to vote for Rick Santorum in order to create electoral choas within the Republican party.
I'm positive that some Democratic voters will show up at the ballot box in Michigan. However, these voters are likely to be partisan Democrats who see Rick Santourum as an easier victory for President Obama in November. These partisan voters are a narrow section of the electorate, but in a close primary contest like the one anticipated in Michigan, they could provide the margin of victory.
But I wouldn't look for a sprint to the polls from moderate Democrats.