Hey sorry I was unable to provide those instructions sooner than you figured it out!
Yep thats what to do essentially :P You can include a portrait wallpaper but I don't as I like to set my background style (back ground 4 all) through cookie.
So in summary for anyone else wanting transparent bars on the new cookie 1.8 (and this is just a temporary fix until someone comes along with an easier method) here you are:
1. Download onto your PC:
http://forum.xda-developers.com/showthread.php?t=503511
2. LOAD... the image
3. Hit auto crop, apply, then close. Choose No for "Set output directory to XXXXXXX"
4. Hit create cab. Make sure only 1 checked box "Delete Wallpaper Manila files after CAB creation". Highlight both softkeybar.png & taskbar.png then hit add files.
5. Choose a destination for the cab so you can find it and hit Create CAB.
6. Softreset after installing the cab.
Yay we're done now you have created a cab to make your current wallpaper have transparent soft&taskbars. Jeez that took a lot of work due to the new 1.8cookie mods
