What software do you own? If you already have PS for Windows, it would be cheaper just to go with a PC. The Dell Studio XPS with i7 is around $1,500 well equipped and the Studio 15 with i5 can be had for around $1,200, also well equipped. The cheapest MBP is around $1,530 and it will have a better screen, but a smaller slower hard drive and it's also a refurbished (though they look and feel brand new). If you went with Apple and Aperture you'd spend less than a PC with Lightroom which would negate the price difference, but you'd still have to buy a faster HDD (well, I would). That would be about $80-$120 more. Again, brining the MBP above the Studio by 5-15%.
Personally, if you're already invested in PC software and knowledge I'd just stick with PC unless you really want to move to OS X - in which case I would not blame you
Screen wise, there are only a limited number of laptops with as good or better screen than a MBP and most of them will cost you almost as much. If you already have an external monitor, then you wouldn't have to worry about that of course. You should look into either an i5 or i7, 6+GB of RAM, 64bit WIndows or OS X and the largest and fastest hard drive you can afford. You should upgrade your memory via a third party like Newegg or OWC. Same for the HDD. Right now the Seagate Momentus XT 500 GB 7200RPM is the hot thing going for hard drives, but any 7200RPM HDD at or over 500GB would be fine. Screen should be at least 1440x900, but 1920×1080 is nicer, but not all screens are made equal. Laptops as a whole are not well suited for serious photo editing, but screens like those on the MBP and some Dell's and Lenovo's are a step above the rest and will do in a pinch (if calibrated).
Edit: Forgot video card. It should not share memory with the OS. If it says Intel, stay away from it. Look for anything with nVidia or Radeon in the description.