T-Mobile has said the buyers of the new G1 Android phone will be able to unlock it 90 days after purchase so it can be used on other networks. But U.S. buyers who think this will let them escape T-Mo’s shaky 3G service are going to be disappointed.

The reason for this is that U.S. carriers in general, and T-Mobile in particular, are in their own world when it comes to 3G technology. To understand what this means, you’ll have to put up with a lot of detail about who does what at which frequencies.

In countries where GSM technology is the standard, that is to say nearly everywhere except the U.S., Canada, Japan, and Korea, things are simple. Voice and low-speed data services are at 900 and 1800 MHz and 3G runs at 2100.

In countries where GSM technology is the standard, that is to say nearly everywhere except the U.S., Canada, Japan, and Korea, things are simple. Voice and low-speed data services are at 900 and 1800 MHz and 3G runs at 2100.

In the U.S., T-Mobile and AT&T both use GSM technologies, but there are fundamental incompatibilities in their 3G services. AT&T runs its 2G and 3G services at 850 and 1900 MHz. T-Mobile's 3G service uses 2100 MHz to transmit and 1700 MHz to receive.

The G1 can handle 2G service at 850, 900, 1800, and 1900 MHz, which pretty well covers the world's markets. But 3G comes only at 1700 and 2100 MHz. That takes care of T-Mobile in the U.S. and everyone else in the rest of the world. But it leaves out AT&T's 3G service.

So the bottom line in that you may be able to get your G1 unlocked, but using it for high-speed data in the U.S. on anything but T-Mobile is a non-starter.