I meant to comment on McCain's speech last night but got distracted by that other speech.
Anyway, the thing to focus on, as Isaac points out, is the attempt to paint Obama as a captive of special interests. I think there are two reasons for this somewhat counter-intuitive charge:
1.) McCain is simply going at one of Obama's perceived strengths--his reformist credentials--and deflecting attention from his own weaknesses on this front.
2.) McCain is subtly introducing race into the campaign. There's some precedent for Republicans invoking the term "special interest" when running against black Democrats--it's meant to stir concerns that the black politician wants to shower benefits on members of his own racial group and stick white voters with the tab.
So, for example, when McCain says:
"Senator Obama might criticize special interests that give more money to Republicans. But you won't often see him take on those that favor him."
I'd guess the point is to evoke not just unions and pro-choicers, or whatever, but also black people. Just as, when Jesse Helms ran against Harvey Gantt, the point of using the term "special interest" was to evoke both countercultural lunatics and black people.
Obviously it's a little tricky, since there's a semi-plausible cover story here. But it seems like a pretty straight-forward play to me. I will no doubt be accused of hair-trigger racial sensitivity for pointing this out...