I believe that when AT&T calculates upgrade eligibility, they do take into consideration how long you've been a customer. I think that for those of us who switched to AT&T in order to buy the first iPhone in 2007 and then bought a 3g in June 2008, that 2-year history with the company is what is causing them to make us upgrade-eligible 18 months into our current contract. Some people with a 3G are posting that they are upgrade eligible in July -- I would guess that those people have a longer history with AT&T.Mine shows December online, or 18 months after I bought the 3G.
It's generally cheaper to keep a current customer than to sign up a new one, so the mobile companies generally do reward long-term customers with treatment that is designed to cause them to stick around.