A recent Clinton ad accuses Obama of dodging a debate at Marquette University in Wisconsin. Clinton accepted the invitation, but Obama didn't answer.
In the 1960 debate between Sen. John Kennedy and Vice President Richard Nixon, which was the first televised presidential debate, most radio listeners concluded that Nixon had won. However, most television viewers said Kennedy won. Nixon appeared without make-up, and looked sweaty and nervous compared to the relaxed, composed Kennedy.
Focus groups have generally reported Obama at a disadvantage to Clinton during head-to-head debates, although he is quite good. The conventional wisdom is that there's little for a frontrunner to gain from a debate with his challenger, and it appears that Obama is showing less enthusiasm for debates now than when he was an insurgent underdog.