Just purchased a an LG L1950S 19 inch lcd (inexpensive, I really don't need a high end model) today to go along with my 12 inch iBook. It's sweet. Performace considered, dashboard and expose are really slow, but expose is handy even if it's slow (as it very very cool looking when you use it when there's no performace issues). I thinking about getting an iCurve and a bluetooth keyboard also to us my laptop in a non-laptop manner.
But, whatever you choose to buy, you'll enjoy the ability you use dual screen.