From Questions and Answers on the jailbreakme vulnerability:
I have an iPhone 3G on which iOS4 ran really slow, so I went back to iOS3. When Apple releases this update, do you think this will be a iOS4-only update, or will there be something for iOS3 as well?F-Secure said:Q: Ouch. Will there be a patch?
A: Apple is expected to ship one as soon as they can.
Q: Is that confirmed?
A: It is. Apple wants to patch this for two reasons: to prevent people from jailbreaking their devices and to protect their customers from potential attacks.