As far as getting "world-phones" that work everywhere (including the T-Mobile USA AWS band at 1700/2100), it could happen pretty quickly.
It's true that T-Mobile USA uses a band (AWS - 2100 downlink, 1700 uplink) that is used nowhere else in the world.
Most 3G/UMTS phones these days, though, are quad-band GSM and at least 2100 band (Band I, the one used in Europe) for UMTS.
The AWS band is "Band IV" in UMTS technology. It uses 1710-1755 on the uplink and 2110-2155 on the downlink.
Band I (all UMTS phones support) uses 2110 - 2170 on the downlink, which includes all of the AWS downlink frequencies.
Band III (1800 band - this is the band that is used for GSM1800 so is supported by quad-band GSM phones) uses 1710 - 1785 on the uplink, which includes all of the AWS uplink frequencies.
Therefore, with a quad-band GSM, single band UMTS phone, all of the frequencies required for AWS are supported. I am not saying that the phones will work with T-Mobile 3G with no modifications; however, it won't require new antennas and other RF plumbing, so the work required (pipe the existing Band III GSM Uplink frequencies over to the UMTS part of the handset to work with the existing Band I Downlink UMTS part already in the phone) is relatively simple (easier than making a quadband GSM phone, actually).
I would expect the phones will be modified pretty quickly and you'll see quad-band GSM, 6-band UMTS (Band I, Band II, Band III, Band IV, Band V, Band VIII) phones soon. Quad Band GSM includes the freqs in Band II, Band III, Band V, and Band VIII, so a 6 band UMTS phone just needs to add Band I (the main European UMTS band) and Band IV (T-Mobile AWS, which is really just part of the existing Band I and Band III).
The current iPhone 3G is quad-band GSM and tri-band UMTS (Band I, Band II, Band V). If Apple wanted to, it'd be pretty easy to make it work on T-Mobile USA. They may not do this, though, until non-US carriers are using the AWS band (probably in South America eventually) so that people are locked to ATT, their partner in the US.
If you want to see the specific bands in a chart, there is a good one at http://en.wikipedia.org/wiki/UMTS_frequency_bands
. I knew the point I was trying to make with this post, but used the chart a lot to make sure I was using the right names for the Bands...