He's been doing this for quite awhile actually. Romney stays ahead of Obama in at least two major polls. Which one? That's constantly varying.
The interesting part is that this is before he's the official nominee, and therefore not benefiting from the full support of the Republican Party. What you generally see is a jump upwards by 5-10%. That means Romney is one candidate Obama really doesn't want to face. Everyone else trails Obama in head-to-head polls by at least 7% (averaging all polls.)
RealClearPolitics - President Obama vs. Republican Candidates
Obama is very good at campaigning -- nobody is better at making lots of pretty promises. So it's not a sure-fire victory if Romney wins the nomination. But it definitely puts the chances of unseating Obama well above average.