Don't know if anyone else has suggested this (I admit to only getting half-way through reading all the posts on this thread) but I had an idea about increasing the efficiency of the dock. I've read quite a few posts from people complaining about the screen real estate the dock occupies, and people resorting to 3rd party apps like the handy F10. What i think would work well (and look damn cool) would be the option to turn your dock into a "solid" 3D rectangular block.
This way when you filled up your dock, or it was getting too large, you could switch on the 3D option and rotate the dock to a free side, and so on. This would also enable you to cleanly divide up the various sides according to different categories of apps' eg.pro/music/work etc. It would look exactly like the current dock when viewing just one side.
This would satisfy both those with limited screen real estate and also those calling for multiple docks. It would also fit in nicely with the Expose look.
Of course they could maintain the dock expansion effect to leave us with a cool "Multi-faceted 3D rotating stretchy dock" (for want of a better description).
Also I can see no reason why it would necessarily need to be limited to 4 sides.
Sorry if I'm just repeating something that someone else has already suggested.