You'll be fine. I've used plenty of unbranded phones on T-Mobile (Nokia N95, N91, E70, SE W950i, etc.) and they've all worked fine.
You'll need to download the web'n'walk settings though. Nokia has got a handy settings wizard on their website, but I assume you can also get them from T-Mobile's website.
As long as the phone does GSM 900/1800 (and UMTS 2100 if you want 3G) you will be fine. The Toshiba you mentioned fits the bill perfectly.