Source: Apple – Support – Discussions – Can’t reset to dock defaults?
What worked for me:
delete the files com.apple.dock.plist and com.apple.dock.db from homedirectory/Library/Preferences and restart Dock. To restart dock either quit it in the activity monitor or enter the following command in terminal:
killall Dock
The dock will restart and it will be rebuilt to the default state.
The Dock will look like it would after a fresh installation of OS X (Leopard, in my case), at its original size, plus all the default icons.