IMO, if McCain "plays it safe" and picks someone that the Republican base pushes on him (like Romney), it will make the Dem's charge of "McSame" more believable and he will fall to the overall aproval ratings of a generic Republican.
McCain must avoid at all costs making this election about Dems vs Reps or he wiill loose big. He has done well so far because he has made the election about Obama the person vs McCain the person. A pick like Romney (and to a lessar degree Thompson) would bring it back to D's vs R's.
If the base can't abide a pick like Lieberman then he should go outside the lines and pick someone from the private sector. Maybe Carly Fiorina would do, though I imagine she has some baggage from her HP days as well.