If McCain wins he will be working with a house and senate with a makeup pretty much like exists today - Dems may strengthen their majorities slightly, but not to a supermajority. Is that congress going to be able to send a powerfully pro-abortion bill (or a powerfully anti-abortion bill) up to the president? I doubt it.
This just seems like a tempest in a teapot. McCain is a pro-life senator, and would be a pro-life President, but he's not likely to be able to do much to advance the issue in either direction. The good news here is, congress is not likely to be able to do much harm, unless there is a much much larger pro-Dem blowout than what is anticipated.
I think the main question here is not so much what kind of a presidency will a pro-choicer be but how many voters will a “pro-choicer” turn away?