I wondered about that too, as we saw that happen in both Wisconsin and Indiana. It sounds like the legislation's already been passed, and all that's required is the governor's signature. Are there different laws in Michigan for legislative quorums?
Correct. Michigan only requires 50%+1 for a quorum, so a minority cannot shut things down by walking out.
One other note -- there are two bills that have to be passed. The one that passed earlier related to public sector unions. The one being discussed now relates to private sector unions.
They're just starting to vote on the second bill (go to house.michigan.gov and you can watch).
It also happened in Texas in 2003 over the vote to approve redistricting.