According to a tweet by one of iPhone Dev Team a Jailbreak is coming.
This is what @MuscleNerd had to say:
@xfsasx jailbreak for all devices at 4.0 is already handled by upcoming @comex release so, good time for iphone4 unlock stuff
@CrAzYe07 the next jailbreak from @comex is like first Spirit..all devices, all bootroms, latest FW, painless install
The Jailbreak may correspond with Apple releasing a Software fix for iOS4 in a few weeks.