Best place to buy an unlocked iPhone?

Craigslist.

overpriced depending where they get them from.
8GB iPhone in Italy with a 2 yr warranty,the second year being local,costs 499€,700$
16GB runs for 800$
no contract whatsoever, no need for a turbosim, sim unlocked,that is.
So if you find some like that on ebay that sell for just a bit over 700$ I say it's a good deal,just make sure they don't need turbosims. 2.2 seems to not work well with them. ...and in that case they would have gotten the phone for less.
I think it's in swiss where you can buy an iPhone without a contract for about 350€, but it would be simlocked, similar story in the UK
In the US you'd have to go in an Apple/AT&T store and sign a contract,buy one for 199$,wait a month,pay that month of service and end the contract,pay the fee.

Craigslist is a good bet if of course you go and see the phone working unlocked without a turbosim.

Like svndmvn said though, alot of places in europe such as here in the UK do them carrier locked but without a contract. Mine was £350 from o2 - carrier locked. I was with o2 already so I had no reason to unlock.

Search around on eBay for sellers with good feedback on iPhone sales. Check for people mentioning the turbosims or e-mail a seller yourself. If you purchase one that does require a turbosim and you were told it wouldn't you have grounds for returning it if you paid via paypal. Just make sure you don't get scammed.

I would think if you wait for the software unlock on Dec 31 that things may get more competitive pricewise....and also more people that are trustworthy may be identified....

I'd go Craigslist as well, because you are able to view the product before purchasing so less chance to get screwed...