There are three main things to consider in a display for graphic work: size, resolution and color depth (not necessarily in that order).
Size should be adequate to working with complex UIs, because professional graphic software uses a lot of screen real estate for panels, toolbars, etc.. This is especially true for video and audio apps, but none of the Creative Suite applications are lightweight when it comes to UI elements. I don't like to work on a display less than 17 inches diagonal, but you may find the next common size, 15.6 inches, to be adequate.
Resolution works hand in hand with size. More pixels allow for more elements to be on the screen. The smaller the screen, the smaller the pixels and the sharper your eyesight has to be for a given resolution. Conversely, a large screen with a relatively low resolution makes everything easy to see, but the space taken up by UI limits your work area. You would be best to try out different laptops to get an idea what will work best for you. My own preference is for 1920 pixels horizontal resolution.
Adequate Color Depth is vital for any kind of accurate graphics work. The modern 10-bits/channl displays are prized by video and film professionals in particular because they can accurately render the widest RGB gamut, but they make accurate color work much easier in any field. These monitors require OS and GPU support for 10-bit output, so are currently worth the extra expense only on the Windows platform. Mac OS still doesn't (as of Mountain Lion) support 10-bit displays. HP and Dell both offer 10-bit displays (HP calls them "Dreamcolor"; Dell has some other marketing name that escapes me at the moment) on their mobile workstations and higher-end consumer laptops.
Of these three criteria, color depth is the one that really makes a difference. Adequate size and resolution are easy to come by.
The only other big recommendation I'd make is to avoid glossy screens. These are consumer displays, strictly. They look sexy because blacks are a bit deeper, but reflections from the glass will drive you crazy when trying to work in anything other than the most carefully controlled lighting (and dark clothing!).
Workstation class laptops are made by Dell, Lenovo, HP and others. Apple doesn't currently have a MacBook Pro model I'd classify as professional grade, even as a replacement for the 2009-model MBP I use on the road.