Does it still say "Full upgrade available…"? If so it's possible you're only eligible for the upgrade price of $399, not the $199 as a full upgrade price (pricing of 16GB model).
For example, the only line on my account with a current contract reads this:
"As a valued AT&T customer, you're eligible for a discounted iPhone upgrade at a reduced discount, along with a 2-year service commitment. You may qualify for a full discount on an iPhone upgrade on 8/21/2012."
My other lines (contract-free) say this:
"You're eligible for an upgrade at a discounted price." Notice the latter does not even mention a date.