I have not played the VP guessing game but I have to believe McCain understands how important his pick is. I presume he will want someone who believes as he does but he has to understand his pick will impact conservative support.
This may be nothing more than a trial balloon to see how Huckabee fares.
Recall that a couple of weeks ago, the hoopla was about Rice being the potential/probably VP selectee, and that rumor fizzled after the responses weren't very positive.