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It's confirmed that if you have a 2G iPhone that is unlocked with BootNeuter (running firmware 2.2.1), you can safely update to OS 3.0 using iTunes (yes just press that Update button).

For detailed and step-by-step instructions see here

This method requires: jailbreaked iPhone 2.x, Cydia, Term-vt100, OpenSSH

Install MobileInstallation Patch from Cydia via “http://www.iphone.org.hk/apt/”, the patch is under section “Tweaks”. If you currently have the hackulo.us MobileInstallation, you ALSO need to install the patch. Don’t worry, you can install appulous apps with this patch.

• In XCode, go to Project > Edit Project Settings.
• Set Base SDK to "Device - iPhone OS 2.0"
• Under Code Signing Identity, set Any iPhone OS Device to "Don't Code Sign"
• Delete the value of Code Signing Resource Rules Path
• Delete all previous build files.
• Go to Project > Set Active SDK > Device - iPhone OS 2.0
• Go to Project > Set Active Build Configuration > Release
• Build. NOT Build and Go.
• Copy MyApp.app to iPhone under /Applications using ssh or whatever method you prefer.

Now open Terminal on iPhone and execute the following code

su -
chmod +x MyApp.app
ldid -S MyApp.app/MyApp
killall SpringBoard //restart the SpringBoard

Run MyApp by touching the icon on SpringBoard.

Update: With the MobileInstallation Patch mentioned above, iPhone 2.2 seems unable to install/upgrade app by itself, you have to use iTunes (8.x) to uninstall an app first, then install/upgrade it.