I'm a huge fan of Sagers, which are essentially unbranded laptops that are the same as the ones sold by the large PC distributors. You can get the same horsepower for a lot less with them, minus Retina and other Apple aesthetics.

http://www.xoticpc.com/custom-gaming-laptops-notebooks-clevo-sager-notebooks-ct-95_51_162.html

The 8GB MBP will be sufficient for nearly everything you want to do. You may want to go above that if you plan on running a lot of memory-hungry VM Servers, such as Exchange, SQL Server, etc. simultaneously though. You certainly don't need a maxed-out Retina system to get started with pen testing.

I'm not a pen tester yet, still learning. I run Backtrack 5 on a crappy old HP hand-me-down from client who didn't want to pay to replace the drive, probably worth about $250. Nothing near your specs. If money is not an issue, go for it. Otherwise, I'd stuff a bunch of RAM into an older model and live with it.

Thanks all! Those are some good insights!

I'm in the same boat. I currently have a 2007 Mac Book pro but when ever I try and load Linux distros (Ubuntu, Arch Linux) on it they do not work with my video card. I'm actually thinking about getting a sager or a eurocom laptop as well, but I'm not sure if they will work flawlessly with Linux, primarily Back Track 5R3 and Arch Linux? Honestly Mac's are nice, I love mine but they need so many work a rounds and hassle to run certain distro's that it is turning me off, especially since I would like to learn the command line more for my  desired profession as a pen tester. So if I were you I would looking into one of those laptops, plus I know you can get a Killer wifi nic that is Atheros chipset, that may work with out the need of a ext. Alfa nic. Good Luck! and if anyone knows about those laptops being great for Linux, please let me know.

So, to quote tturner, I ate my own dog food. I just picked up one of these: http://www.xoticpc.com/sager-np9150-clevo-p150em-p-4341.html?wconfigure=yes

I did a few upgrades, such as display, memory, and disk, but it's still a pretty kick-ass machine with the standard config. I'm dual-booting Windows 7 and Linux Mint (Cinnamon), and both are running as smooth as can be.

I had spend a few minutes messing around with the Linux wireless driver, but all the other major hardware was instantly recognized and configured. I haven't tested everything yet. There may be other issues with random hardware, such as the fingerprint reader or card reader, but I'm really not concerned with those items.

That's a nice rig. I'm in the market for a new laptop myself as my aging 17 inch brick is no longer beefy enough and far too heavy. I've always liked the Sagers and may have to pick one up. I have a core i5 Windows slate now, but nothing like a real keyboard/laptop format.