The iPhone Dev Team has released an updated version of Redsn0w; which will address the battery drain and overheating issues after hacktivation. This works hand in hand with SAM or the Subscriber Artificial Module developed by @sbingner.
UPDATE: If you have an AT&T iPhone, you can officially unlock it using this.
Follow these instructions for the fix.
1. Download the latest version of Redsnow 0.9.6b6 (download here)
2. Jailbreak your iPhone 3G or 3GS using this guide. (Do not do the ultrasn0w unlock yet).
3. After jailbreaking your iPhone, launch Cydia and add http://repo.bingner.com to the sources.
4. Install “SAM” and “SAMPrefs”
5. Start Redsn0w 0.9.6b6 again, and select “Deactivate” option. Unplug your iPhone from the computer after the deactivation is completed.
6. Go to your iPhone, navigate to Settings > SAM and tap on “De-Activate iPhone”
7. Connect your iPhone to your computer then launch iTunes, this will allow iTunes to officially activate your iPhone. You should see the message “iPhone is activated”.
NOTE: If iTunes says invalid SIM, you will need to manually select the appropriate carrier instead of “Automatic” and click on an SIMID.
8. Now do the unlock via ultrasn0w 1.2.
Good luck and let us know how this goes for you!
If this worked for you, please consider buying us a drink!