There are various reasons why you’d do this, and with t-mobile letting you get it without a contract you might want to try to make it work with T-mobileWeb for only $5.99

First here are the basic settings:
Menu->Settings->Wireless Controls->Mobile Networks->Access Point Names->T-Mobile US
I input the following settings:
Name: T-Mobile US
APN: wap.voicestream.com
proxy: 216.155.165.050
port: 8080
Username: Not set
Password: Not set
Server: Not set
MMSC: unchanged from original (http://mms.msg.eng.t-mobile.com/mms/wapenc)
MMS Proxy: 216.155.165.050
MMS Port: 8080
MMC: unchanged (for me it was 310, I believe it is based on your location)
MNC: unchanged (for me it was 260, I believe it is based on your location)
APN type: Not set

Here are the comments from the original commenter about what WORKS, and what doesn’t.
- 3G should work
- Gmail push will not work
- POP3 and IMAP will work (so gmail will work through that way)
- Google maps doesn’t work
- And Android Market DOES NOT seem to work.

For the apps that don’t work you can usually use them when you’re in the wifi network (well not maps, but you get the idea)

G1 News, Misc, News 3g, Access Point, android, apps, google, Maps, Mms, Mnc, Mobile Access, Mobile Networks, Mobileweb, Msg, Names, Port 8080, Proxy, Settings Menu, T Zones, t-mobile, Wap, Wireless Access, Wireless Networks