For those of you that are using T-Mobile in the US and are having trouble getting service using yellowsn0w, we found a tip made by the commenter “Richie Rich” on the iphone-dev blog. I personally tried this after some mishaps and made it work. I’m using T-Mobile in Chicago and it definitely saved me some waiting around for a different or newer solution.

And again, yellowsn0w is a beta product. The iphone-dev team guarentee jack squat except pure “l337ness”.

Here’s a few steps if you can’t get it to work:

1. After installing yellowsn0w through Cydia, keep your T-Mobile SIM inserted.
2. You must reset the network settings.  This is probably the most important step.
3. Reboot your iPhone.
4. Turn Airplane mode on (Settings > Airplane mode: ON).  Turn it off after a couple of seconds.
5. You should see “searching” near the top, and this is when the glory happens.

Let us know if this worked for you either in the US or anywhere else in the world.  Keep in mind that T-Mobile USA uses a different 3G network than AT&T, so US iPhone 3G WILL NOT work on the T-Mobile 3G network.  You must DISABLE 3G network from the Settings pane.