Is McDonnell the only veep prospect that's widely being talked about with military experience?
I looked at the reasonable (~1% and up) prospects on Intrade and it appears to be so.
Just for fun, here are the Republican Senators with military experience:
Scott Brown
Thad Cochran (MS)
Michael Enzi (WY)
Lindsey Graham
James Inhofe
Johnny Isakson (GA)
Richard Lugar
John McCain
Pat Roberts (KS)
Jeff Sessions (AL)
Roger Wicker (MS)
Mark Kirk (IL)
Dan Coats (IN)
Of those, Brown, Enzi, and Isakson were in the National Guard.