While there was software available to change the IMEI on the first generation IPhone, there is no such software to do this on the 3G model. The answer comes do to the fact that the two phones use two completely different technologies.
The first generation IPhone operated on the GSM network only. Many people wanted to change or cloak the IMEI on the phone to make it look like a different brand of phone. The reason behind this was to get a less expensive cell phone plan, since the plan on the IPhone was more expensive.
3G IPhones operate on the UMTS network when they are in the 3G mode, and only in the GSM mode when their coverage on the UMTS network is poor. The hardware in a 3G IPhone does not allow for the same hack that allowed people to crack the first generation IPhone.
I never say that something is impossible, but, if there is a hack its existence has not hit the main stream.