Yes, you can use cingular software on a tmobile phone. You have to delete the vendor.xml file (located in: C:\Program Files\Common Files\Research In Motion\AppLoader on my WinXP machine). It will work fine except you may only have the default list theme. If that's the case, see jibi's sticky on themes.
If you want to just stick with TMobile's software you can get it here without a TMobile number. The latest is 188.8.131.52: http://software.se.t-mobile.com/