I had the L900 too and i was able to unlock the phone by just sending a mail to AT&T customer service, i only had to leave the phone IMEI and nothing else. I never was an AT&T customer and by then i was living in Brazil and i bought the L900 on ebay. The process took a few days and AT&T people gave me the unlock code.
That was before the US phone unlocking ban law, now that law is temporarily overturned, should be less restrictions and difficulties to unlock US phones.