Well, there is another difference between the msgbox function and popup method as was pointed out to me by several people. Msgbox only displays the first 1024 characters of a dialog whereas popup displays a lot more (I haven't looked to see what the actual limit is although I can certainly fill the screen and more besides). Taking yesterdays script and adding this immediately before the call to msgbox
szMessage = ""
for i = asc("a") to asc("z")
szMessage = szMessage & String(100,i)
Msgbox will display
but Popup will display
So now you know, but please, no more emails if there are other differences!