Well, autofocus always happens wide-open on modern camera's/lenses.
The sweetspot of the kitlens is around 35mm where it performs best from f/8 on. At 18mm you should stop it down to f/11 if there's enough light.
The lens isn't really that bad as everyone suggest if you know how to use it.
I'm thinking about trading my 17-40 for the new 17-55 IS, but not before prices have come down to at least €750-800. In the meanwhile I think I might pick up a 24-70 2.8 L or a used 28-70 2.8 L, but that's a different storie.
If you don't need a lot of wide-angle, the Tamron 28-75 f/2.8 is a good option. It costs <€300 and the example I had was really sharp, even wide open. In fact, at 70mm it might've been sharper than my 70-200 2.8 L. But 45mm (28x1.6) is almost a standard-lens, so it depends on your shooting-behaviour if the lens is usefull.
I think it's best to just use it for a while and see where your priorities are. Tamron has also just come out with a 17-50 f/2.8 and Tokina is about to release a 16-50 f/2.8, it's gonna be interesting to see how those will perform.