Lifehacker has a guide: http://lifehacker.com/5572003/how-to-downgrade-your-iphone-3...
Along with other changes, many of which do not affect the iPhone either, the biggest change in Snow Leopard was to switch all binaries and the default compilation target from IA-32 to AMD64. You shouldn't expect the Leopard -> Snow Leopard upgrade experience to translate to ARM-based mobile phones.
PS: I upgraded an iPhone 3G to iOS4, some apps are slower and others even crash more often, but the home folders and the "Cellular Data" setting in Settings->General->Network make it up for me.
It progressively got slower. Keyboard response ground to a halt. It would take 3-5 secs for the keyboard response to show up on screen.
It became an useless paper weight.
I tried restore to 3.1.3 a few times and I would give up at the 1015 error and restore back to 4.0, which was probably a "clean" install. It didn't fix it.
The primary feature I was interested in was the orientation lock which is not available in iPhone 3G. Threaded email is nice, but not a deal breaker, same with folders.