Here's what Wikipedia says:
Botanically, a tomato is the ovary, together with its seeds, of a flowering plant: therefore it is a fruit or, more precisely, a berry. However, the tomato is not as sweet as those foodstuffs usually called fruits and, from a culinary standpoint, it is typically served as part of a salad or main course of a meal, as are vegetables, rather than at dessert in the case of most fruits. ...The term vegetable has no botanical meaning and is purely a culinary term...[Tomatoes are not] the only culinary vegetable that is a botanical fruit: eggplants, cucumbers, and squashes of all kinds (such as zucchini and pumpkins) share the same ambiguity.
Okay? So now I never want to hear again, as long as I live, this phrase: "Tomatoes are not really vegetables."
Update 2009-08-11: More than you ever wanted to know about what is a "true fruit" here.