This is the latest from McCain. They clearly are stepping up the attacks. The problem is that almost none of it is true.
For instance, it is true that Obama served on the board of the Annenberg Challenge with Bill Ayers. It is also true that the Annenberg Challenge is an organization funded by GOP and former Nixon administration official, Walter Annenberg, and also has the GOP member Arnold Weber on its board. So, Obama gets smeared as a terrorist for working with a Republican funded organization.
It is also true that Democrats, including Bill Clinton, were also involved in pushing for more lax regulation. However, McCain now alleges that a law passed in 1985 requiring underwriting minorities is responsible, which seems bizarre to anyone except those who want this to some outsider or minority's fault.
They also put the blame for deregulation completely on the Democrats, which seems to conveniently forget that most of the legislations were passed by Republican Congresses to resounding cheers from the GOP. The "ownership society", i.e. housing for all, was in fact Bush's specific campaign promise which helped propel him to victory.
Finally, the NY Times reports that McCain's justification for linking Obama to the housing and financial crisis is a line from Obama's speech where he said, "Subprime lending started off as a good idea - helping Americans buy homes who couldn’t previously afford to." The problem is that this was the first line in a speech in which Obama explains the source of the mortgage problem and outlines a plan demanding more transparency and regulation. Most people who'll see the ad, of course, won't know all this. So, the ad will likely be effective.