In the interests of research, I installed iOS4 on my iPhone 3G and immediately regretted it. First off I couldn’t connect to GMAIL, it needed a lot of coaxing – deleting accounts, remaking accounts, then rebooting eventually made it work but this isn’t what I expect from Apple. Speed comparisons with two other iPhone 3Gs showed that it is slower in everything it does. Usually just a second or two but occasionally much slower. Sometimes it initially appears quicker, as in Google Maps where it instantly displays the last map shown. But then after a quick check on the GPS location it redraws the correct map and loses out again to OS3.1.3

Apple didn’t warn us that many Apps would fail to run with the new firmware, including some potentially crucial apps such as Skype – which no longer works! That was very disappointing.

Why would you want to install iOS4 on a iPhone 3G anyway? There are only two visible benefits – folders and threaded email, neither of which makes up for the loss of responsiveness, so it just isn’t worth it. BTW with the iPhone 3GS there are many benefits to iOS4 and if there is any slowdown it’s unnoticeable.
Use the information here to restore your iPhone 3G back to OS3.1.3 – it’s a laborious business but not too difficult, no thanks to Apple.