A while ago, I was watching an N900 user trying out an iPad. It was interesting to see that he had substantial difficulty figuring out how to move back in nested screen hierarchies. He was actually trying to press on the black border around the screen to go back, because on the N900 all dialogs are designed so that clicking anywhere in the area "behind" the dialog takes you back.
IMHO, that "buttonless back button" is perhaps the best thing about the N900's UI design.
IMHO, that "buttonless back button" is perhaps the best thing about the N900's UI design.