Oh dear oh dear oh dear
When we say that "AT&T uses 850MHz and 1900MHz", we mean that there are two distinct bands it can use, each of which contains both the uplink and downlink. That is, "850Mhz" means that both the uplink and downlink are around "850MHz", and "1900MHz" means that both the uplink and downlink are near 1900MHz. Similarly, when we say "European networks use 2100Mhz", we mean that both the uplink and downlink are near 2100MHz. However, when we say "T-Mobile uses 1700Mhz and 2100MHz", we mean 1700Mhz for the uplink and 2100Mhz for the downlink. This is not two bands, it is one, but the uplink and downlink are a fair distance apart. The idiot of an analyst quoted in this article does not understand this, and because he has heard "2100" in both the description of the iPhone and of T-Mobile's network, he has assumed they are compatible, which they are not.
In my mind it is better to refer to T-Mobile's network as using the "AWS" band (as it is referred to by US regulators) as this avoids the confusion. I wouldn't actually say that T-Mobile's setup is all that weird - it's just different from the world outside the Americas. The AWS band has actually been allocated throughout most of the Americas (excluding Brazil, which has allocated bands mostly on the European model, as have a few places in the Carribean). There are or will shortly be networks using that band in Canada, Mexico, Chile, Argentina, and more before long.
Now, Apple's new iPhone 4 works for 3G/UMTS on the 850Mhz, 900MHz, 1900Mhz, and 2100MHz bands. That is every band in the world on which UMTS has been deployed *except* the AWS network used by T-Mobile. I think from this we should conclude the precise opposite of what the idiot analyst said: that Apple does not intend to offer the iPhone on T-Mobile for at least the next year. Apple could probably have fairly easily added support for AWS/T-Mobile if it wanted to - Nokia has managed to produce a phone supporting all five bands for UMTS and I suspect if they can so can Apple, but for now Apple has not done this. I suspect the issues are legal: it is widely believed that Apple signed a contract giving AT&T exclusivity for five years. Contracts are renegotiated all the time, but this likely likely has not been, so we probably have two more years of AT&T exclusivity.