If you have to just modify the iPhone 3G baseband to 2.28 without updating the firmware to 2.2 or 2.2.1, this is the easiest method to follow! The main reason for only upgrading the baseband to 2.28 is to achieve software unlock and still preserving all the jailbreak apps and appstore apps on the iPhone 3G. This has nothing to do with downgrading the baseband from 2.30 to 2.28 though.
Steps required in order :
1) Download ICE2_02.28.00.eep, ICE2_02.28.00.fls, BBUpdaterExtreme from this rapidshare link

2) Make sure your iPhone 3G is jailbroken and …

Go ahead and follow this if you have a jailbroken iPhone 3G (Firmware below 2.2.1) and don’t want to mess around with the unlocking tools for 2.2.1 firmware.
Follow the following steps for both Windows and Mac Operating systems :
1) 2.2.1 Firmware from Apple iPhone 3G updates the baseband to 2.30 disabling the software unlock.
2) Make sure you have jailbroken your iPhone 3G Firmware below 2.2.1 using QuickPwn 2.2.
3) Now,if you have access to a Mac, feel free to download Pwnage 2.2.5 and build a custom firmware. Windows users might want …

Those of you who want iPhone 3G 2.2.1 firmware with 2.28 Baseband and Cydia, download both the files below and join them using hj split.This works both on Windows and Mac. 2.2.1 may not be an important upgrade but it definitely puts and end to accidental upgrades to 2.30 baseband.

Please make sure your iPhone 3G is already pwned (you should have Cydia installed!).
Launch iTunes (Windows or Mac) and restore your iPhone to the downloaded Custom Firmware. (Hold Shift key (Windows) / Opt key (Mac) whle hitting the restore button in …

UPDATE : Custom Firmware comes to the rescue.
Apple sneaked in a worthless firmware update (2.2.1) to thwart the attempts of Dev Team to unlock the iPhone 3G via software means. Please stay away from the update till a patched firmware without the baseband update is released. I repeat, baseband cannot be downgraded and since Apple patched the baseband exploit, 2.30.03 which comes as a surprise with 2.2.1 is not unlockable by any means at this time. Save yourself some headaches and enjoy your iPhone as you are till yesterday! Any …