These days there were quite a lot of questions regarding iOS 4.0 and the possibility to activate/decode the phone after an update to the jailbroken version of iOS 4.0 GM, so I decided to make this article to explain what you can/can't do with iOS 4.0 GM at the moment.
First of all, if you have baseband 05.12.01 you CANNOT decode the phone regardless of the firmware you restore to, whether it is 4.0 or 3.0, there is no decoding software that can decode that baseband or baseband 05.13.x from iOS 4.0 GM.
Those with iPhone 3G who update to iOS 4.0 GM will update their baseband and by default will have to wait until the Dev Team releases a decoding program, you cannot give the baseband back at the moment.
Those with iPhone 3G who update to iOS 4.0 could activate the phone using redsn0w 0.9.5 beta 5 but I don't know for sure if this is possible because I don't have an iPhone 3G to test. Those with an iPhone 3GS cannot activate the phone by updating to iOS 4.0 because the update process to a jailbroken custom firmware requires a restore to 3.1.2 and if you need activation then you most likely do not have SHSH for firmware 3.1.2 saved .
I hope I covered everyone's questions with this article.
