Nestor D.L. emailed TechPinas this question yesterday:
"TechPinas, can you give me an advice on where I should go to jailbreak my iPhone 4?"
Although George Hotz aka GeoHot, a popular iDevice Jailbreak enthusiast (if not THE jailbreak progenitor), has released a photo of a jailbroken iPhone 4 via his blog, as yet, there's no software available that the public can use to jailbreak iPhone 4.
Jailbroken iPhone 4, Credit: George Hotz
UPDATE as of August 7, 2010: It's now possible to Jailbreak an iPhone 4. Click on link to find out how.
Keep in mind that jailbreaking an iDevice does not only void its warranty but can also leave it vulnerable to viruses, worms and spyware attacks.
Thanks for the inquiry.