In System Preferences, you have a few options in positioning the Dock. You can set it to reside along the bottom, right, or left edge of the display, and you set it to remain visible or hidden until you need it. By default, there's about a half-second delay for the hidden Dock to appear. I've never liked this delay; I frequently wait for iPhoto to spring to life, and there's always a delay when opening iTunes, so why do I need a delay for the Dock? To be sure, it's an admittedly short delay, but I'd like my MacBook Pro to feel as peppy as possible.
There's a simple Terminal command that kills the delay for the hidden Dock to show. Open Terminal and enter this line of code:
defaults write com.apple.Dock autohide-delay -float 0 && killall Dock
Now, the Dock will appear instantly when you mouse to the edge where you have it hidden.
If you'd like to return to the default, half-second delay in showing the Dock, run this command:
defaults delete com.apple.Dock autohide-delay && killall Dock
The change takes place immediately and does not require you to log out and back in again. According to LifeHacker, this Terminal command works only with Mac OS X Lion. I don't have an older Mac to use for testing, but I can report that it worked with Mac OS X Lion.
While you have the Terminal open, you may also want to check out.