I also got my 3GS on --or very soon after -- launch day at the discounted price (can't remember which), but I am showing that I am upgrade-eligible. Others, who seem similarly situated, are reporting different experiences.
I'm wondering whether one's upgrade-eligible date depends, in part, on one's monthly bill -- those paying for more expensive plans getting earlier dates? (I have a Family Plan with three users on unlimited minutes.) I'm not saying that this would or would not be a good thing, but simply that this might explain the discrepancies.
AT&T has clarified this afternoon that the "before the end of 2010" deal means that you get the low price on the iPhone 4 if your upgrade-eligible date was to fall before the end of 2010, not simply if your contract expires before then. That is consistent with the fact that I am showing eligible, even though my contract presumably runs into summer 2011 -- on the basis that I triggered a new 2-year term with the 3Gs in summer 2009.