In iPhone OS 3.0 beta, the Settings->General->Nework->Cellular Data Network is empty, so we cannot change any APN settings. So if you have to manually change APN setting, you cannot use any 2G/3G network.

To solve this problem, you can visit this website:

http://unlockit.co.nz

using your iPhone Safari with Wifi. Following the instruction, enter your APN information, and the website will create a profile and installed to you iPhone.

unlockit.co.nz main screen, click "Continue" to edit

Enter your APN settings and click "Create Custom Profile"

The website will create a profile, confirm to install the profile to your iPhone