My ex and I negotiated it in our divorce - he'd pay X amount of child support and I would claim the kids on my taxes. Like Sierra said, if you want to make a different arrangement, all you have to do is fill out a form.
Another thing to think about (and my ex and I have had several discussions on this topic):
Typically it's the C parent that is on-call 24/7 - if the kid is sick, the C parent takes the time off work and deals with the doctors. While the non-C parent is meeting friends for drinks after work, the C parent is running home to the kids. While the non-C parent is entertaining a boy/girlfriend in their apartment (if you know what I mean), the C parent has to actually go somewhare else and pay a babysitter to do it. Etc, etc, etc...
So if you think you're getting screwed on your child support, add up an hourly wage of $6.25 (which is cheap), then multiply it by 24 hours. Comes out to $150 per day. That wage doesn't include food, shelter, clothing, school supplies, etc. My ex tried that old "I'm paying for more than half of the kids' essentials and it's not fair!" I replied, "Well, then let's crunch the numbers..." and that usually shuts him up.
If you are a non-custodial parent, in essence you're paying someone else to raise your children full-time so you don't have to. I'm certain that statement will make some of you non-Cs defensive and you'll come back with some story about how you fought to retain custody and the ex had a better lawyer or some other blah blah. But we can address that subject if it comes up.
See what you started, HippieChick?