Just installed iOS 4.0 Gold Master on my 3G and jailbroke it. With all the iOS 4 goodness disabled on the 3G, there's even more reason to jailbreak. A quick change to a Springboard plist file and you get the home screen wallpapers and native multitasking!
Maybe it's just from a clean restore, but I swear it's faster than 3.x
Followed these directions, super easy:
http://pixelatedgeek.com/2010/06/ios...for-iphone-3g/