Like a few others here, my wife and I also first heard about geocaching from a Law & Order: Criminal Intent episode aired in 2005.
But I didn't actually decide it was time to dig in to it until I saw this picture of a lake on Flickr a month or two ago. The caption includes "Found while geocaching..." and it was then that I remembered the L&O:CI episode and figured I needed to really see what this was all about.