A tethered jailbreak solution for iPhone 4 is already available. Just as you would know, this solution updates your baseband as well which means users who have unlocked their iPhones may get their phones locked to the original carrier once again. Of course, an UltraSn0w unlock is on its way and by most counts should get released by tomorrow. But if you are one of those restless souls who want to get it done now, then here is some quick solution.
Folks at FSM blog have been able to build a custom firmware that makes use of an unofficial PwnageTool bundle to enable a jailbreak/unlock solution that preserves the baseband of your iPhone 4. The complete instructions are here. But before you proceed, make note of their disclaimer that this is compeletely untested and is not for the newbies.
If you do not want to risk getting your phone bricked, wait for UltraSn0w to release. It is just a matter of one more day.