Hactivating an Apple iPhone can be a tricky business specially when your iPhone 3G/3GS is having the "iPad 6.15" baseband for retaining software unlock. After, repeated failed attempts to have an Apple iPhone 3GS running iOS 4.0 with baseband 6.15 hactivate using custom IPSW and redsn0w the following worked for me.

Hactivating your iPhone does have it's side-effects and one of them is that you are not able to use iPhone Push Notifications, the problem arises because Apple identifies devices via unique certificates and validate them on PUSH servers - However, this problem is now been taken care of by new Cydia application "Push Doctor" made possible by it's creator anethema who has found a way to generate these unique certs en-masse to supply a proper fix for all the hacktivators out there. Installing the app will simply download and inject unique certs into your phone's keychain making them uniquely identifiable by PUSH servers enabling the functionality properly.