iPhone 4 was recently jailbroken and now it is unlocked too, thanks to the Dev Team. It cannot be easier, no PC/Mac required, only the internets!
All you need are the following :

A Jailbroken iPhone 4 / 3GS,3G (it should be activated, that is a pre-req for online jailbreak)
A WiFi connection
That is pretty much it!

Step by step (best practices) :

Launch Cydia and search for ultrasn0w [if not found, then add repo : repo666.ultrasn0w.com in Cydia]
Install ultrasn0w and reboot your iPhone 4 when prompted. Turn 3G off  (Settings -> Network -> Enable …

iPhone Dev-Team delivers again in style! All the basebands for iPhone 2G, 3G released till June ’09 are unlockable by software means, thanks to the super famous yellowsn0w. Pwnage tool and QuickPWN are being worked upon to be ready for the D-day! Please throw away the turbo sims and refrain from buying into their propaganda! The entire jailbreak community will be able to upgrade to the latest in Apple’s offering – the revolutionary iPhone 3.0 software which managed to sneak in the most awaited features – MMS, tethering, Copy/Paste and …

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 …