From the Boston Globe:
Overall, for the six networks that aired the program simultaneously, the spot had a household rating of 21.7% (meaning that 21.7 percent of all households watching television were tuned to the spot) in the top 56 local markets. In Boston, the rating was 25.7 -- meaning that just over a quarter of local households tuned in...Baltimore market had the largest TV audience, with a household rating of 31.3, while the Portland market had the lowest household rating: 14.2.
That's a little better than I expected. The Globe goes on to give numbers of the recent debates for comparison. If we assume only Obama supporters and indies watched this thing, and these same people watched the debates, then it would imply that only around a quarter of the debate watchers were McCain voters.
FWIW: I was at the gym at the time and about 1/2 the screens were tuned in to it. My wife overran her limit because she was too interested in the show to pay attention. So she's in pain this morning thanks to Obama. :)