With Sn0wbreeze 1.6.1 for Windows, you can now create custom firmware files of iOS 4 which are already jailbroken just like what PwnageTool 4.0 / 4.01 can do on a Mac. Currently, only iPod touch 2G (non-MC model, and already jailbroken on anything other than Spirit) can be jailbroken on the newly released iOS 4. Both the iPod touch 2G (MC model) and iPod touch 3G requires a new exploit which is currently not available. So if you haven't already jailbroken your iPod touch 2G with Redsn0w, you can do so with sn0wbreeze 1.6.1 for 4.0.
Redsn0w jailbreak tool has been updated to version 0.9.5b5 which brings full support for iPod touch 2G, along with optional support to enable multitasking and wallpapers on iOS 4.0 (formerly iPhone OS 4) on iPhone 3G and iPod touch 2G which otherwise are disabled by Apple.