was on Apple's website today and under the technical spes, I noticed that the iPhone 4S supports the UMTS/HSDPA/HSUPA 2100 band (which I believe T-Mobile uses).
Does this mean I can get 4G or 3G on T-Mobile if I get an unlocked iPhone 4S? (using the prepaid plan)
The 2100MHz 3G band is currently only used in Europe and Asia. T-Mobile USA uses the 1700MHz band for their 3G and 4G networks. They're planning on putting the 1700MHz band in use for LTE eventually and reuse the 1900MHz band (which the iPhone 4S does support) for their 3G and 4G (not LTE networks). The problem is that they don't have enough chunks of the 1900MHz spectrum, so they won't be able to deploy 3G or 4G on it in a lot of areas.
So, to simply answer your question, you won't be able to use 3G on the iPhone 4S on T-Mobile at the moment, until they refarm that 1900MHz spectrum, which they might never do in some markets, such as NYC for example.