It looks like iOS5 will finally let Apple put a stop to some of the jailbreakers they've been after. The latest beta prevents users from downgrading to earlier versions of iOS. Meanwhile, untethered jailbreaks are effectively out the window.
The just released limera1n promises to jailbreak iOS 4.0-4.1 on iPhone 4, iPhone 3GS, iPad, iPod Touch 4G, and iPod Touch 3G. It's Windows only for now, with Mac & Linux support coming soon.