It was Dubya who said he wanted to see 'Democracy' sweep across the Arab States (though he probably wasn't thinking of Bahrain, Saudi and Qatar). The problem for America, is that most of the real despots in the region, they have supported for 30 years.
So they are torn between two coflicting opportunities. They can see Gaddafi is weak and they would be happy to see the back of him. But that would encourage uprisings against their chums in Saudi. So the US just turn up for the barrel shoot, but they probably don't want Gaddafi to be removed, just serverely weakened.
If he stepped down or someone else took over, the US could start doing business with the Libyians without seemingly getting its hands dirty by sucking up to Gaddafi (like Blair eventually did). The Americans would quite happily deal with Libya its present unpredictable, barmy, brutal leader was changed.