Unless he can convince Petraeus, Romney needs to pick somebody with no ties to Washington or else he runs the risk of Obama successfully tying him to "the do-nothing Congress."
Governor McDonnell has never served in Washington, and has been Governor of Virginia for about as long as Spiro Agnew was Governor of Maryland when Nixon picked him to be his running mate in '68 (And Nixon was, more than anybody else, an ardent believer in the philosophy that a running mate is only good for not doing harm to the ticket; no more or less). Unless the vetting shows otherwise, McDonnell is the ultra safe choice who helps seal the deal in two states pretty early on (i.e., native Virginia and neighboring North Carolina).
We can only hope Romney's Nixonian Do No Harm philosophy leads him to pick the next Spiro Agnew.