It will work on other networks, it just won't use 3G except with T-Mo (if it's a T-Mobile USA handset) unless those providers use 1700/2100 for 3G, and I believe it's only T-Mo and Wind that use those frequencies.
i have an AT&T bold 9700 unlocked with 3G working in Kuwait , Does that mean T-Mobile won't work ? i has to be this or that ?
probably wont work. if your carrier uses the same 3G frequencies that at&t does, i doubt they would also have coverage on the tmobile frequencies as well. not many carriers use those frequencies.