My 2007 edition of Travelers Guide mentions some of Michigan's peculiarities regarding vehicle transport of long guns but doesn't mention the 30" overall length issue.
Do the 2009 or 2010 editions address this? I guess it's time to update. It sure helps to have this compendium of the carry and transport rules. Of course, having it only helps when I remember to review it before we cross state borders.
We visited one state a couple weeks two years ago and I happened to be looking at Travelers Guide and realized, "Holy Crap!, I wasn't allowed carry in the truck anywhere in that state!" California is one more special state, too.
Watch out, plan ahead, be mindful.