If you bought an iPhone 3G at launch last year you can upgrade to an iPhone 4 this year without paying a typical penalty. From AT&T:
To let current iPhone customers quickly upgrade to iPhone 4, AT&T will make its best pricing immediately available to any iPhone customer who would be upgrade eligible for a discounted device by the end of 2010.
How generous. It sounds to me as though AT&T is about to get some competition in the US and wants to lock in iPhone 4 customers for two more years.