On my past 2 cruises, I've flown down the morning of. I guess I'm one of the few that has a hub as my local airport (Newark Liberty Int'l; hub of Continental Airlines), so I've had direct flights down to Miami and Fort Lauderdale. I've made it without a problem.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)