I dont know if this helps you....but cingular (which is basically t-mobile here in NYC) offers an 18% discount for corporate accounts.
There business discounts aren't that great in comparison but considering were there regular rates are anyway i am not suprised.
the best tmobile discount would be to cancel tmobile so you are paying nothing for it. and join verizon.
Wow!!!! That was alot of help (insert scarism) I would say try calling a local store and asking for a business rep. Jack