I got a replacement from an Apple store and it was basically pre-activated. I just swapped sims and it did kind of an over the air check or something because after about 30 seconds it said "iPhone now activated". Didnt have to do anything through iTunes.
OP, did you get yours from the online store or from a retail store?
I got both iPhones from the apple online store. The new phone was not a "replacement". It was a brand new phone. The way they did the replacement is they just shipped me a new one, and set up a return for the old one. When I switched the SIMS, nothing happened. The old phone was still activated and connected to the same phone number, and the new phone was still unactivated.
Now, the new phone is activated with it's new SIM in it, because I used iTunes "replace an existing phone with this iPhone" option, but my old phone would still activate after I restore it in iTunes. It does that automatic activate thing after restore. So right now, i've unplugged it from the computer before it had the chance to do that, so my old phone now looks like a brand new, unactivated phone, but if I plug it into my computer, it'll activate on the same phone number that my new phone is currently on.
What the bleep do I do?
edit: I'm on the phone with AT&T. They say that it shouldn't have stayed activated, and that I don't have two numbers or two bills on my account. They say they're calling Apple, to ask if there's anything they have to do aside from shutting my phone down. Then they'll shut it down, so I can ship it back.
edit 2: My old SIM card is now dead, for good. They killed it.

Now everythings good.
And I still have one number, and one billing. Apple doesn't even want the old SIM card back. They said to ship it back without the SIM in it.
Also, of random note, my new iPhone has a much better screen than the last one. The contrast is richer. It looks almost glossy.