Pew apparently has Romney leading Obama 49-45.

[technical support]

It is extremely unlikely that 21% of the nation's Democrats changed parties as a result of one debate. So there is a fair chance that the Pew poll is an outlier that undersampled Democrats and oversampled Republicans. For comparison purposes, two other national polls hav also been done entirely since the first debate. Rasmussen has it as a tie at 48% to 48% and Gallup has it at 47% to 47%.