Just like some online retailers do, many airlines will refund the difference if the price goes does after you buy your ticket. I was listening to a Personal Finance podcast from Wall Street Journal today and they recommended Yapta for tracking ticket refunds. Just register your ticket information and they will email you with the refund info if the price goes down and you are eligible for a (partial) refund.
They mentioned that Jet Blue pays out most of refunds. Not sure why, whether they hike the prices a lot of have the best refund policy