As far as platform goes, even moreso today I think it's worth looking at the Windows vs. Mac debate. Now, if your budget is < $1000, Windows is pretty much your only option for a portable.
That said, if you are willing to spend $1200-$1600, a lot more options open up, on both the Windows and PC side:
Windows: I would stay clear of Dell, but that's my personal opinion. Having a Dell Small Business account, I've bought probably 10 or so Dell machines since 2000, a mix of laptops and desktops, and the more recent ones I had nothing but problems with.
HP makes good stuff, I recently bought my mom a 15" HP laptop and it's pretty nice. I'd focus on HP or Asus if you are looking at a Windows laptop. Something with at least 4GB of RAM, 320gb HDD, SD card slot, and a Core 2 Duo (or a new i5) at the least.
Mac: I personally have a 15" MacBook Pro from 2009. You can't get Blu-ray, but they come standard with 4GB of RAM, built in SD, and at least 320GB HDD. The 13" in my opinion are just too small for photo editing unless you use an external monitor (my wife has a 13" MBP, and I can't stand the screen).
It depends what your needs are. If you have a large collection of Windows software already, it's probably worth it to stay with Windows. In that case, look at HP, Asus IMO as far as hardware goes.
If you don't mind starting over, the MacBook Pro's are superior hardware to anything you can get from the Windows world (granted you can run Windows on a Mac, I run Win 7 on mine).
DISCLAIMER: I'm a Windows developer by trade, but use Mac's for my personal laptop and photo / music production. My house is a hybrid
