Can’t help but wonder if this was a cynical attempt to appear above the fray while getting the negative ad out to a wider audience than he would have been able to afford:

Former Arkansas Gov. Mike Huckabee (R) told reporters Monday that he was pulling a negative ad designed to target his chief rival in Iowa, former Massachusetts Gov. Mitt Romney (R).
Then he showed the ad to a room packed wall to wall with reporters.
When asked if it is hypocritical to make an announcement about not running negative ads, and then show a negative advertisement to dozens of members of the national and local media, Huckabee said he had to show the ad to prove its existence.
“I want to show you that we were fully prepared,” Huckabee said.
The former governor said the ad, which hits Romney on the issues of taxes and abortion, was set to start running in Iowa Monday at noon.

Romney’s response?

Romney spokesman Tim Albrecht said Huckabee is trying to have it both ways, by withdrawing the ad while still showing it to the news media.
“To say one thing one minute and then turn around and show an attack ad to reporters the next will, obviously, leave folks with a very cynical view of Mike Huckabee and his message,” Albrecht said.