This happened to mine the other day (just a couple months out of warranty!). It's not that it wasn't responding at all; it would sometimes respond if I clicked pretty hard. (I mostly used the "tap" method anyway, but it get's annoying when your using the magnifying glass in Photoshop.) This led me to believe it was a hardware problem, rather than, as many here have suggested, a software problem.
Then today I pushed pretty hard at the bottom of the trackpad and "CRRRACK!" the glass layer on the trackpad broke!
Now the trackpad works perfectly, but the danger of getting little splinters of glass in my finger means I'm going to have to replace the trackpad. I'll swing by an Apple store and see how much they want to over-charge me to do this before ordering a replacement (and the tools) on e-bay.