Bill is spot on... the closer you are, the trickier it gets.. for the shots I posted above...
f/4.5 at about 2-3 feet away
f/2.8 at about 1.5 feet away
f/3.5 at about 4 feet away
when I'm doing true macro work (flowers, food... etc.) I always use a tripod and cable release and usually have to manual focus. If you auto focus really close up wide open, the lens will just keep going in and out trying to focus.
Here's an example... this was shot very close... inches away.. at f4.5 6.0sec. No way it would have been sharp without a tripod. And it was also manually focused at this close range.

For babies, I find it works really well at about a 2-3 foot range. It's great for letting me get really close and interact.