I thought this was discussed many times before, but AT&T's upgrade eligibility is actually pretty straightforward. You have to look at how much you spend each month on YOUR line. Remember that texting plans do not count.
Actually I have not seen this information in the past couple of months of reading MacRumors, so thank you for posting it. It makes a lot of sense as to why my eligibility dates seem to be a lot sooner than so many other people's, e.g. currently my iPhone 4 is eligible for full upgrade 11/5/2011 but I bought it in November 2010 on a 2-year contract.