According to the AT&T rep I spoke with any upgrade other than paying full price will reset upgrade eligibility. Like you I was considering paying the extra $250 but when I was told it would push my next upgrade eligibility back 18 mos. I reconsidered.
I'm eligible next month so I'll see what I can sell my iPhone 4 for then. If I can make it close to a wash buying the 4S I'll buy.
So be aware if you pay the $250 now and then want the next years iPhone too you'll have to pay $250 again. If you wait until you are eligible for a subsidized 4S then you'll just pay the $250 for next years phone.
This is what I was wondering about, if thats the case I will just wait for my upgrade to be available and only pay the $250 once.
I may even skip this gen, wait for my contract to be up in a year and switch carriers for the next gen, I am starting to really dislike the way AT&T does business.