Riginslinger, you are right about the Obama decision being quite strategic. I don't know what he intentions and motives were.

The Wiki entry on the MI primary does, however, mention this as a possibility:

"While Biden, Edwards, Obama, and Richardson officially left the Michigan primary in deference to the DNC's rules regarding primary dates, a few political analysts claimed that they unofficially left in order to deprive Clinton of the momentum that she would have gained in Michigan."