The iPhone Dev-Team recently has released a new version of RedSn0w for Mac and Windows. RedSn0w uses the Limera1n exploit to support more devices and adds the ability to restore custom firmware ipsws. The crew promises that now RedSn0w will work with the iPhone 3G, 3GS, 4, iPad, iPod 2G, 3G, and 4G (Not iPod 2G-MC yet).
You can download RedSn0w 0.9.6b2 from here.