This is interesting
Obama was wrong to state that "100 percent" of McCain TV ads have been negative. According to a survey by the Wisconsin Advertising Project at the University of Wisconsin, 73 percent of the ads McCain has aired since the start of the campaign have been negative while 61 percent of Obama's ads have been negative. According to a Nielsen survey, both candidates had roughly similar numbers of negative ads beteen June 3, when the primaries ended and September 7. During that period, the McCain campaign ran 76,192 negative ads against Obama while the Obama campaign ran 75,246 negative ads against McCain.
It's possible Obama was referring to recent ads aired by McCain. The Wisconsin Advertising project reported that "nearly 100 percent" of McCain ads between September 28-October 4 were negative.
Final Presidential Debate Fact Check - Fact Checker