new iphone 3g- can't use 3g internet because i need to change 'apn'??

I'm not sure if the 3G phone is suitable for T-mobile. They do the iPhone but never did the 3G version, only the 3GS and iPhone 4

Have you contacted T-Mobile to see if your data plan is compatible with the iPhone?

The iPhone has all of the proper APN's settings for all official carriers already programed into the OS, that's why you don't see the screen to change these settings(fact is, most carriers don't want you messing with these settings anyway). You should only see that screen if using a sim from a non-official carrier in an officially unlocked phone. You first course of action is to contact T-Mobile. You could also try restoring your phone with the T-mobile sim installed in your phone. Doing this will also install an updated carrier file, if needed.

We have a PAYG iPhone 3G on T-Mobile without any problems at all. Have you sync'd the iPhone with iTunes since the unlock?

SHARK!

I have exactly the same problem
Got an iPhone 3G of a friend which was officially unlocked by O2 so i could use my T-Mobile PAYG sim
Everything was working fine - able to make calls, send texts etc - had added credit so i could access the internet.
I've been with T-Mobile a while so decided to swith to a contract so i could benefit from free internet
As soon as i did i was no longer able to connect to 3G - no longer see the icon in the tope left of the screen
Have restored the iPhone many many times - once i've restored i can access the APN settings but that makes no difference - when i accept a carrier settings update the tab dissappears but i still can't connect

It seems to be an issue with T-Mobile contracts, which is extremely annoying that i've now tied myself into a 12 month contract that doesn't work
Have spent ages on the phone to T-mobile customer service - may give Apple a call but i'm pretty sure it's T-Mobile issue

I have this vague notion that your provider should be able to reset something at their end which forces the phone to pick up the settings again. They perform this reset and then when you sync the phone with iTunes it picks up the 'new' details. I'm sorry I can't be more explicit - it's just something I'm sure I read somewhere on these forums.