If Romney is out this is a huge mistake by McCain. Palin has zero name recognition. This reaks of McCain desparately trying to pander to the female voters. It reminds me of the Mondale / Ferraro ticket and will probably be as successful.
Picking Palin would definitely make it harder to go after Obama for his “inexperience”. McCain would lose a lot of credibility on that front.
The big differences are that in 2008 the name recognition can be won very quickly, and Palin is inherently far far more appealing than the sour and tedious Ferraro ever was.
Once Palin is well introduced to the country she will do great. She doesn’t need to lead the ticket after all, she just needs to help here and there at the margins. Ferraro couldn’t add anything but Palin can, IMHO.