I found this neat little tool to make the taskbar transparent. Download Transparent TaskBar, Transparent TaskBar 1.0 Download
1. After install you'll find a little rectangle on the right lower corner of your desktop.
2. There is a little slider in there to set your desired level of transparency on the fly.
3. After you are satisfied, you rightclick the rectangle, click exit, and forget about the app.
It doesn't need to run on boot.
The transparency however is maintained after reboot.
If you need to change or disable it just run the app again.
Screen shot of it running with RTM on my old laptop: