Dickerson sums it up well:
...Ayers was an accepted member of the community when Obama first got to know him. He worked for a university and was well-respected in education reform circles. If Obama was being ambitious, it was within community standards. To condemn him on this point, you'd have to condemn all of Chicago. As a political argument, that takes you pretty far afield for the voters you're trying to convince.
Something else I haven't seen from the McCain camp: An example of someone who took the supposed high road and refused to work with the guy. Did anyone in Chicago do this? Would make for a crisper case than, as Dickerson says, indicting the entire Chicago establishment.