Europe and Asia use different bands for 3G than North America uses.  This guide will look at which areas use which bands and the results of mis-mtching bands and areas.

Band I   WCDMA 2100 MHz Africa, Asia, Europe
Band II  WCDMA 1900 MHz North America, South America
Band III WCDMA - 1800 MHz Unassigned
Band IV WCDMA 1700/2100 MHz (AWS) United States (T-Mobile 3G)
Band V  WCDMA 850 MHz Asia, Australia, Brasil, Canada, South America, United States
Band VI WCDMA 800 MHz Japan
Band VII WCDMA 2500 MHZ
Band VIII 900 MHz WCDMA
Band IX 1700 MHz WCDMA Japan

From the above list you can see that if your device has only 2100 MHz WCDMA UMTS ability it will not function as a 3G device in North America. So, when you slip your T-Mobile SIM into it you are going to get EDGE speeds at best. An example of this would be the Nokia 6290. This is a very capable Symbian 60 UMTS 3G phone for Europe with 2100 MHz WCDMA capability for Europe. When deployed in the US it receives both T-Mobile and AT&T Internet at EDGE  speeds. The same applies to your Japanese device operating on 1700 MHz. It will not function on the T-Mobile 3G system because it will not be able to communicate on the 2100 MHz portion of the AWS band.

So, if you are looking for a 3G device for use in the US here is what to look for:

AT&T

850  MHz WCDMA
1900 MHz WCDMA

T-Mobile

1700/2100 MHz (Advanced Wireless Service) AWS

AWS utilises frequencies in both the 2100 MHz and 1700 MHz bands. The 2100 MHz band in Band I is not the same area of the radio spectrum as the 2100 MHz used in Band IV. The mobile device transmits in the 1710-1755 MHz band; the base transmits in the 2110-2155 MHz band. It is for this reason that WCDMA 2100 MHz devices from Europe will not function, they do not have the 1700 MHz band enabled.

A Quadband GSM phone will have 850 MHz and 1900 MHz GSM bands listed in its specifications. It can receive the frequencies but in the GSM format. It will not recognize nor will it process information in the WCDMA bands in those frequencies unless it is designed specifically to have it. In that case the specifications for the device will list it.

These phones will provide 3G speeds on the indicated networks. This is a very abbreviated list. There are many more phones out there that will get the fast Internet speeds. The AT&T list is much longer because I've limited the T-Mobile list to those phones that T-Mobile offers. The AT&T list includes phones capable of reception on the AT&T WCDMA frequencies and may or may not have been offered by AT&T.

AT&T

Apple iPhone 3G
HTC Titlt 8925
LG CU320
LG CU400
LG CU500
LG CU515
LC Shine
LG Vu
Motorola A845
Motorola V3xx
Motorola Z9
Nokia 6500 Classic
Nokia 6555
Nokia 6651
Nokia E51
Nokia N75
Nokia N78
Nokia N81
Nokia N95
Palm Treo 750
Pantech Matrix
Samsung A737
Samsung A767
Samsung Access
Samsung Invision
Samsung SGH-ZX10
Samsung A747
Samsung A767
Sony Ericsson W760a
Sony Ericsson Z750a

T-Mobile

HTC G1
Nokia 3555
Nokia 6263
Samsung SGH-T639
Samsung SGH-T819
Sony Ericsson TM506