That depends when and where they got them.
According to reports Monday in Gizmodo and Ars Technica, citing AT&T sources, U.S. customers who bought an iPhone from Apple (aapl) or AT&T (t) after May 27 will be allowed to trade it in for a new one with no additional charges (beyond the usual 10% restocking fee).
Customers who purchased an iPhone on or before May 27 are out of luck.
In the U.K., O2 is offering its iPhone customers a better -- if more complicated -- deal. You have to know what "top-ups" are to understand the terms offered on O2's website here. It's a little clearer as MacWorld UK explains it:
"Existing iPhone customers on the £45 per month contract will be able to upgrade their existing model to the new 8GB iPhone for free on July 11 by visiting the O2 Website. As we reported, you will need to sign up for a new 18 month contract. Existing customers on the £75 per month contract will receive the 16GB iPhone for free.
Current iPhone users on the £35 per month contract will be required to pay £99 to upgrade to the new handset and sign up for a new 18 month contract. They can, however, move up to the more expensive £45 per month contract and get the iPhone for free." (link)
No word yet on trade-ins from Orange in France or T-Mobile in Germany.