Pete Warden has a post up about how “discovery” is different from “search.” We’ve included “next-level discovery” (embodied by companies like Lijit and AdaptiveBlue) as a defraggy topic since day one, and Pete does a great job of teasing out some nifty threads.
The main points that Pete makes include:
1. “Search is goal-oriented, discovery is about the journey.”
2. “Search is solitary, discovery is social.”
3. “Search is about universal answers, discovery is customized. ”
4. “Discovery is a background task.”
ahhh, there’s that whole “implicit” thing again. 
Good stuff! I’ve never given much thought to the difference between search and discovery, but I’d bet at least have of my searches are for the purpose of discovery rather than simple, goal-oriented search.