It should not be an issue since the warranty starts as soon as the iPhone is activated on AT&T. Since you have already done that you are good to go for warranty. The individuals that hit the warranty issue have either voided the warranty or the iPhone was never activated on AT&T. If you got a not very bright Apple Genius, just try another store or use the mail-in service.
Apples warranty obligations for this hardware product are limited to the terms set forth below:
Apple, as defined below, warrants this Apple-branded hardware product against defects in materials and workmanship under normal use for a period of ONE (1) YEAR from the date of retail purchase by the original end-user purchaser (Warranty Period).
Dave