It appears we have an issue unlocking this phone since release. There is either a delay with the release of codes from AT&T or they are purposely holding out until a specific date to release the codes to unlockers. This delay period is unknown at this time. It could be tied in with their exclusivity period or could be tied into their $100 off promo and white variant release around the 21st.
In the meantime I thought I'd complile a list of unlockers who advertise being able to unlock the AT&T Lumia 900 and maybe some of us that don't mind trial and error can go through and eliminate the ones that do not work..
1) Unlock All Cellular - $64.99 (24-72 hrs)
UNAVAILABLE WHEN I TRIED BUYING IT
2) World GSM Codes - $59.99 (2-3days)
RESULTS: Money refunded. No reason.
3) Cellphone Unlocker - $24.99 (up to 72 hrs)
4) Mobile Xpress Canada - $30CAD (3-5 days) Btw, this code is bestseller on their list
5) Cut Your Sim - $79.99 (2-7 days) Page claims the code is "In stock"
6) Cell Unlocker - $28.99
RESULTS: No code available from their servers. Money refunded
7) Unlock That Phone - $39.99 (2-5 days)
RESULTS: Money refunded.
8) GSMFast.net - $50 (7-12 days)
9) Shady little place in Chinatown NY City