Open Terminal.app (/Applications/Terminal)
Enter the following command:
defaults read -g com.apple.mouse.scaling
That should return the current speed of your mouse tracking as an integer. Mine was 5 and I want to update it to 10. Here is how I did it...
To change the speed of your mouse tracking you can simply run the following command:
defaults write -g com.apple.mouse.scaling your_mouse_speed
where your_mouse_speed is the speed you want to set to your mouse tracking.
Then you'll have to Log Out or restart your Mac to see the changes take effect.
UPDATE: As reported by the reader Squished Spider, if you update your settings using the approach above and perform any changes to the Mouse settings using Mac OS System Preferences later then the system will override everything you did and it will revert them back to the slow mouse behavior even though you've set it to the maximum speed.