Two established sources indicate that both members of Michigan's 2007 right guard platoon will not be returning for fifth years due to medical issues. (Yes, medical issues beyond Mike Barwis.)
Though neither exactly shone in '07, losing both leaves Michigan's offensive line in a precarious position. Justin Boren and Steve Schilling return. Mark Ortmann is a good bet to start at one of the tackle spots. Past that it's all question marks; Michigan's inability to get more than two offensive linemen in the '07 recruiting class doesn't help.
Purely speculative, but based on buzz I would guess Michigan opens 2008 with a line like so:
|Left Tackle||Left Guard||Center||Right Guard||Right Tackle|
|Mark Ortmann, Jr*||Justin Boren, Jr||Dave Moosman, So*||Steve Schilling, So*||Perry Dorrestein, So*|
At this point looks like Michigan's 2008 offense will have zero senior starters.