I just got the iPhone that operating system version is 4.0, i can insert the card 2G. However, the system version 3.1 after upgradeof the 2G can not be used only with 3G card. I want to know that how to downgrade the iPhone Operating System 4.0 to operating system 3.1. Please suggest. Thanks in advance. 

Can I downgrade from iPhone OS 4.0 to 3.1
There is no way to downgrade3.1 to crack and, but with the magical kit (well-known hacker GeoHot) You’re done the whole series the perfect escape, by the use of jailbreak. Apple shouldannounce at this conference the main functions of iPhone OS 4.0 update, and launch Beta version of the SDK (software development kit) for third-party software developers to prepare ahead of time, but not immediately ordinary users to publish iPhone OS 4.0. 

Re: Can I downgrade from iPhone OS 4.0 to 3.1
You need to jailbreak phone activation, you can install the program, such as codling many practical tool to load up their phones. You can contact including Video Crack the iPhone, sending MMS and run more than one Application Program. All of this can be unlocked after the iPhone. 

Re: Can I downgrade from iPhone OS 4.0 to 3.1
Connect iPhone to your MAC or PC, and then in the iTunes device list select the iPhone. If iTunes can not correctly detect the iPhone, the iPhone will need to put DFU (Device Firmware Upgrade) that is device firmware upgrade mode.
In accordance with the following steps to iPhone into DFU (thisstep is only normal for iTunes can not recognize iPhone users):
• Connect iPhone to your computer.
• Turn off iPhone.
• Press and hold the key and the HOME key to open and insisted for 10 seconds (as accurately as possible!!).