Raise your hand if you're tilting your external Display
weaksauce: I have a 24" hooked up to my 15" MBP. The Monitor is up and to the right of the laptop. It does not bother me to have different height monitors because my workflow is such that I use keyboard shortcuts to move windows to the other monitor.(sizeup and cinch are the best apps that I have purchased) If you have/are getting a mac look into sizeup and then map the keys according to this scheme: 
http://zacharypinter.com/2009/06/15/keyboard-driven-window-m...with one addition from me: ctrl+option+command+k should center the window in the screen. This is useful for making the window the left half of the screen with ctrl+command+j and then centering the smaller window. (sometimes pages are too hard to read on a large monitor but you don't want to have the window off to the side.)Edit: I personally would not like positioning the monitor above the laptop because I use the 24" much more than the 15" and always looking up would be extremely bad for ergonomics.