Just unlocked my s710a through Cingular!!
I called Cingular customer service, told them that I had purchased model "T637" in the past and had it unlocked through them. The service manager replied, "that is against our policy, and that it probably did not happen." - I replied, "I disagree, the customer service rep asked for my IMEI# and the provided my NCK code. The service manager then changed his story and told me that he could unlock my s710a as long as it was not still active, I told him that I had purchased the phone recently and was moving to Europe and would be changing providers, and felt that b/c of the cost of the phone that I did not wish to buy another one once over there. He immediately took down the IMEI# and provided the unlock code. Elaborate stretch of the truth but Cingular did give me the unlock code. Good Luck.