there's a rumor of a text convo between harbaugh and andy moeller (his college roomate/teammate, the son of ex-michigan coach gary moeller, and the current o-line coach of the browns) where moeller tells harbaugh it's time to come home, and harbaugh responds with "we're getting the band back together, we're on a mission from bo" (bonus points to anyone who can name the movie reference).
this text message has been circulated among the interwebz and i can't vouch for it's authority. but there are a lot more ties to michigan for harbaugh than just taking a job at a big time college program.
now in the end, it doesn't mean he takes the job (although people on the michigan end remain optimistic). but i can't see harbaugh solely using michigan for leverage. he would be alienating a lot of people in doing so, including close friends and former teammates, and most importantly, his dad, who (reportedly) has done a lot of the behind-the-scenes legwork between the michigan camp (the current AD played under jack harbaugh) and the harbaugh camp.