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.
The simplest way I knew of to commit these settings once I had altered and saved .GlobalPreferences.plist was to log out and log back in.
[robg adds: We talked about mouse scaling in this hint from late 2001, but we've never covered trackpad scaling. As with the previous hint, if you happen to use the Keyboard & Mouse System Preferences panel to change the speed setting again, you'll have to re-do your edits to get your faster speed back.]

