If you mean the splash screen at startup - yes, it will still say Cingular. You can't change that. The network name at the top of the screen will reflect T-Mobile or whatever network you are really on.
now comes the real question, should I wait for the T-Mobile 8800 or buy an unlocked one now!?
That's your judgement call to make... If you have a pending upgrade available it would cost less to get it through TMO. However, if you can't or don't want to wait you can just buy now and unlock.