Well, it's bound to be a lot better than it used to be in Snow Leopard. Because now you can make apps full screen. But still... I tend to like a large work area when in Photoshop or Aperture (that's kind of like Lightroom I guess). It helps you get to grips with what you're doing I think. It certainly means you get a better view when zooming in to work on small sections.
Bear in mind, as well, that a 15 inch MBP will undoubtedly run CS5 programs better. They're quad core after all. I wouldn't count on the rumour of a 15 inch Macbook Air though.