Figured this out earlier today:

I highly recommend backing up these registry values before modifying anything. If you don’t know how to do that, I don’t recommend that you even attempt this right now =)

1. Make sure your Zune is not plugged in and your Zune software isn’t running
2. open up regedit by going to the start menu and selecting “run”. Type regedt32 and hit “OK”
3. Browse to HKEY_LOCAL_MACHINE\System\ControlSet001\Enum\USB\        EDIT: With Zune Software v2 you may have to edit .....\ControlSet002\.....
   
4. Search for “PortableDeviceNameSpace”. This should be contained in the Vid_045e&Pid_0710\########_-_########_-_########_-_########\Device Parameters within the above …\USB\ The ##’s listed here will be numbers and letters specific to your Zune
5. Change the following values:
   • EnableLegacySupport to 1
   • PortableDeviceNameSpaceExcludeFromShell to 0
   • ShowInShell to 1
6. Plug in your Zune, and make sure the Zune Software starts up.
7. Hopefully at this point you can open up “My Computer” and browse your device, though it does NOT show up as a drive letter.

Right now I can access it both as a harddrive and sync through the Zune Software.

EDIT2: Using this hack, two Zune icons appear in the Windows Explorer. To remove the one of them go to (as above): Device Parameters\WpdMtpDriver\UseWpdPrivateInterface and change its value from "1" ==> "0".

EDIT1: Zune Software needs to be open all the time. There is an authentication mechanism and if you close it you can only view the contents of your Zune. (Not even play them). 

By pressing the "Start Sync" in Zune software, temporarily you get write access to files and folders during sync. So drag and drop files using Windows Explorer into Zune in the proper folders (Music, Video, Pictures). You should avoid placing or deleting any files from the root directory of Zune.

Nice hack ZuneBoards!  Check some screenshots here.

Let us know if you have tried it and works / doesn't work in our  user Forum .