That celcom officer must be ignorant by the fact that you have ALREADY successful using the phone with Maxis. Thus, there is NO NEED for unlocking the iPhone 3G.
In your case, you have confirmed that you are using officially Singtel set (although there times we need to verify this).

Yes, there are times, you are unable surf the Net with celcom 3G even tho' you can see the 3G signal.
All you need is to REBOOT the iPhone and surf the Net again. Somehow, the iPhone lost that 3G connection.
The only condition is to enable the "3G" which you have done that. (Can you see the 3G or E icon?)
Also you need to set the "celcom3g" APN (which you have done that too).

ONLY leave username and password blank.

Have you tried rebooting?

This post has been edited by Edoras: Jan 10 2009, 11:12 AM

3G speed is lower than 3.5G (HSDPA)