- Open up the Terminal
- Type defaults write com.apple.Dock orientation top
- Restart the Dock process. You can either log out then back log in again, use ProcessViewer (in /Applications -> Utilities), or using "ps" and "kill" if you know how to do such things.
- Open up the Terminal
- Type 'defaults delete com.apple.Dock orientation'
- Restart the Dock process as above.
[Editor's note: I know this is an oldie, but as far as I can tell, it's never actually been directly posted before! If you'd rather not use the Terminal, just use TinkerTool, which can move the dock and make the changes take effect immediately.]

