I thought sidekicks used a special apn for data, so if you put their sims in another phone, you can only get voice, maybe the sim tells the n900 no data available and the n900 for some reason doesn't properly unlock the wifi, or maybe because the t-mobile apn for the sim only allows sidekicks, it shuts off data in a wierd way that causes the n900 to not open wifi.

That's all I could think of off the top of myhead.