Jailbreaking your iPhone is now as easy as visiting a web page. After two days of happiness all around it starts to sink in that there is a major security issue on the iPhone iOS 4 right now that could allow hackers whatever they want with your iPhone.
The security hole JailBreakMe is using to jailbreak your iPhone is related to the PDF viewer and embedding fonts on the mobile Safari according to MacRumors.
Apple actually needs to update iOS 4 asap to close this hole. It will be a hard decision for you to either update or loose the jailbreak functionality. Unless you use this instructions ( found on MacStories) on your jailbroken iPhone that enables a warning when a PDF is loaded. Jailbreakers are actually more secure than normal iPhone users right now, Apple your turn.