As of January 1 2007, the minimum wage for Florida employees is $6.67. However employers can deduct a "tip credit" of $3.02/hour from the wages of tipped employees, lowering their legal minimum wage to $3.65, with the assumption that tipped employees will earn at least enough in tips to increase their wage to the state minimum, $6.67. Tipped employees are taxed on tips, whether or not they actually receive them. And Claire is exactly right -- if servers were paid a living wage in the US, we'd see an increase on the menu prices to cover that.
When I eat at a restaurant, I just automatically consider a 15% tip as part of the bill, for just basic service -- the server took my order, brought my food, asked if it was ok and fixed any problems if it wasn't, refilled my drink, brought my check. Anything at all above and beyond that, even just a cheerful personality

, is likely to increase that tip. Very, very rarely would I leave less than 15%, the service would have to be truly horrible -- so bad, that I would probably speak with the manager. And I would only consider lowering a tip for a problem specifically with the
service -- it is not the server's fault if the kitchen messes up my order, the other tables aren't bussed, the food isn't what we were expecting. If my server does their job, they will get their tip.
I know that in other countries, the customs are certainly different -- and the servers are usually paid a fair living wage, and not dependent on the whims, generosity, and fairness of their customers. We got some weird looks when we tipped in Ireland, and our guide told us that tipping was not customary or expected, even though it was appreciated.
