The rule of thumb that I learned (if I remember correctly) is:

If it contains seeds, it's a fruit.

According to this rule, tomatoes, which many people believe are vegetables, are in fact fruits. (However, tomahtoes are snobbish vegetables ;)

Great - now I just confused myself. Does this mean that coconuts are vegetables?? I don't remember seeing any seeds in them.

Wow Cig Highwind - I had no idea! It's a good thing our Supreme Court is spending its time on critical issues!

UPDATE: StrawberryFrog says "a coconut *is* a realy big seed, monocot style." This makes sense because if you plant one you get one palm tree.