Just out of curiosity, would it even be worth watching a 1080p movie on a 15" screen? Would you even notice the difference, unless you were right close to the screen?
The difference between standard and high definition video becomes less as the screen size gets smaller. When watching a good quality DVD on my 17" MBP from afar it looks more or less as good as watching a Hi-Def trailer. On my 50" TV though it's a completely different story.
If you wanted the same resolution as the high def 17" MBP on a 15" screen then you'd end up with a DPI of 150! That's insane. Text and icons can already get too small on my 17". With uncertainty over the Resolution Indepence features of Leopard, you'd need to get your glasses out for a 15"