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The data plans are carrier specific. Thus, the 200Mb plan (which is AT&T only) does not apply to T-Mobile.
 
If T-Mobile offers the plan and it can be used with any smart phone, then yes. Remember US T-Mobile does not have any iPhone plans and you will be limited to EDGE speeds. Because it do not yet have the iPhone and the iPhone does not support the US T-Mobile 3G frequency.
 
Doesn't T-Mobile still have the $10 unlimited data plan for non-smart phones? When I was with T-Mobile back in the day I had that plan with my iPhone.
 
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