I've known some people for whom the fastest trackpad speed setting still seemed sluggish. You can manually change the setting to whatever you want by modifying the .GlobalPreferences.plist file, located in your user's Library/Preferences folder. Notice the dot before the name -- this means that the file is hidden in the Finder. The easiest way I know of to open it is to open Terminal and type:
open ~/Library/Preferences/.GlobalPreferences.plist
Once you've got the file open, changing the setting for com.apple.trackpad.scaling to something higher will make the trackpad more responsive. For example, a setting of 5 is noticeably faster than the fastest setting available in System Preferences, and a setting of 20 would be uncontrollably fast. You can also change com.apple.mouse.scaling to alter the speed of mouse tracking.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051224140406391