Really, they let me upgrade from 3G to 3GS to 4. And I waited to get 3G few month after it originally came out.

See this article on from 2009: http://news.cnet.com/8301-13579_3-10266781-37.html

update: Based on my recollection, with 3G and 3GS, AT&T only asked for 1 year contract, not two. When new iPhone was released, sometime in June or July, anybody with contact renewal through October was marked as elegible to sign new contract, and received contract pricing on the newly released model if iPhone.