How do I undo my iOS 4 upgrade for my iPhone 3G

I recently downloaded the new iOS 4 update for my iPhone 3G and now my phone is the slowest thing I have ever held in my entire life. On a daily basis I want to throw it out the window.

That said, please tell me there is a way to undo this update. I fear that if I "restore" using iTunes it will reset my phone to factory defaults, meaning the only update available would, again, be iOS 4.