Not even once.
During the most important speech Romney's given so far, he didn't say a single word about the one time in his life that he's ever been elected to anything. That's rather extraordinary admission by omission that he doesn't see his record as something that he can run on.
It's not hard to see why Romney feels that way. Massachusetts ranked 47th in the nation in job creation under his leadership and he left the state with a massive debt. If he'd run for a second term, he would have been defeated, with good reason.
Romney's biggest legitimate accomplishment was Romneycare, which President Obama took national. But now Romney is against Obamacare, so he can't talk about Romneycare.
Romney's failure to mention his gubernatorial experience really was a telling moment in his campaign, a vivid illustration of the fact that he believes his campaign depends entirely on convincing voters that President Obama has been a failure. As Greg Sargent has been arguing, Romney's approach to that task has been to blame Obama for the failures of the Bush years—to make the public forget that President Obama inherited an economy in freefall.
As depressing as it would be if Romney were able to pull off such an argument, I don't see it happening. His argument is transparently false, and neither the Obama campaign nor grassroots progressives are going to let him get away with making it. As tiring and annoying as it can be to constantly battle back Romney's lies, it's not particularly difficult—because the truth is not on his side.
I suspect that deep down, Mitt Romney knows just how tenuous his position really is. If he were really confident in his own claim to the presidency, he wouldn't be trying to blame Obama for Bush. He'd be out there selling his own achievements. The fact that he's not tells us everything.