Nuelectronics sell a shield that does this but I figured I would learn more by building my own. Theirs has a keypad and some other features but the screen is rather small.
http://www.nuelectronics.com/estore/?p=9
Long story short, LCD Smartie is configured to send output using "matrix orbital" commands, which the nuelectronics folks have written a sketch for. I just took the sketch they provided and hacked it to work with a 4x20 LCD (the shield's is only 2x16)
Hope that helps.
-fs