Plank is a lightweight dock. I tried it out on an old laptop running Xubuntu 12.10 and liked it. Eventually I decided that plank could take the place of individual app launchers and the window buttons plugin on the xfce top panel. So I got rid of those.
Screen space is tight on the laptop. I looked at the blank space in the middle of my top panel, looked at plank taking up space on my screen, and thought, why not have plank share the top of the screen with the top panel? Well, it doesn't work because plank and the top panel both want to be on top. They compete with each other.
I found a workaround. I put a small, transparent panel on each side of the top of the screen, and I have a transparent themed plank in the middle. It works with no conflict, great! Unfortunately the transparency means I have to paint a fake panel at the top of my desktop picture. This makes it difficult to swap desktop pictures, and changing to themes that specify different color panels is not easy either.
I am posting a screen shot of my top panel area so you can see what I mean. If anybody is interested I can post the plank theme and settings I use. It would save a lot of trouble if I could configure the xfce top panel to stay underneath plank. That seems unlikely.
I see someone has written an xfce panel plugin for another dock called DockBar X.
An xfce panel plugin for Plank could be the answer I am looking for.
I have learned that there is an existing panel plugin that gives most of the functionality of plank-dock.
This works for me in xfce 4.10, Ubuntu 12.10. I am marking this thread as solved.