Just one day after iOS 4.1 was released, a new jailbreak was announced.  As I understand it, the new jailbreak is a bootrom jailbreak, which means that it will be impossible for Apple to patch with a software update.

So, theoretically, it will always be possible to jailbreak the devices that are using the current hardware.

As of today, the jailbreak is still being tested, but hopefully it will be released sometime in the next few days.

Source: http://www.redmondpie.com/jailbreak-ios-4.1-iphone-4-3gs-3g-ipod-touch-4g-bootrom-pwnagetool-4.1/