MuscleNerd: @xfsasx jailbreak for all devices at 4.0 is already handled by upcoming @comex release so, good time for iphone4 unlock stuff
Comex is the developer behind the jailbreak for iPhone 3GS, 3G, iPad and iPod touch.
The question isn't if, but when, the i4 JB will be released. Since an iOS 4 update is due within a few weeks, it would only make sense for the devs to wait for that to hit before showing their cards. But I'd expect it soon thereafter though.
Why jailbreak? One major reason, for me anyway, is to run a personal WiFi hotspot on my iPhone 4.
Tip: Redmond Pie