As far as I know, various Unix versions, as well as Win7, will run just fine. No artificial performance penalties, and Apple is currently doing precisely NOTHING to disallow installation of alternate OSes. This has been the case for years.

Except getting macbook to boot anything else than OSX or Windows requires bunch of stuff that I can only describe as 'hacks'. At least it was like so couple years ago (http://mjg59.dreamwidth.org/12037.html) and I haven't heard that things would be different today.

Try booting a Linux install USB on a Mac sometime

Running Linux installed by this method on a (2008) Mac right now. Have done the same on recent (2012, 2013) models, too.

Macs have been using EFI boot for a few years, so the problem most have probably faced is trying to boot in BIOS emulation mode (which I've found to be iffy on pretty much every EFI motherboard I've come across).

Arch Linux has particularly good support. :P

I did, a few months ago. Booted both Fedora 19 and 20 Beta on a 2012 13" rMBP. Worked decently well, especially with how the version of Gnome included in F20 knows how to properly deal with high DPI displays.

Hasn't worked for me at all no matter how I create the USB, working with a 2011 MBP. Tried literally every method, and several distros.

I've had good luck booting Fedora (at least 19 and 20, and I think 18) on Macs.

Other distros don't seem to work with Apple's EFI implementation. To boot other distros, I've had good luck with the refind[1] bootloader. Granted, I have only been booting live USBs, not installing on the internal drive.

1: http://www.rodsbooks.com/refind/

You can and you won't and it is. Performance issues are from the fact that other OS's don't support the hardware properly.

Well, considering the fact that Apple made a program called "Bootcamp" which simplifies the process of installing Windows on your machine, I'd say other OSes are officially allowed by Apple.

In fact, I have Win7, OS 10.9, and Arch Linux on this computer (early 2013 MBP Retina)

The IT guy in my building claims "MBP's are the best Windows laptops on the market." I'm probably not in a position to say whether or not it's true, but it seems to be at least a defensible position.

Which building?

If/when I get one, I'm going to run it without the shiny exterior as much as possible. That thing is too shiny and I much prefer the aesthetic of PC boards and parts on a Pro machine. (Then again I run my Windows tower with the sides off too, though that's because I was too lazy to hook up the power button and front USB ports.)

It has a sensor that disallows running it with the case off. Disabling the sensor would void the warranty. Not saying nobody will do it, because I'm quite certain someone will (or already has?).

But you'll probably have to actually spring for your _own_ MacPro to mod the sensor.

Thermal design might rely on a laminar air flow inside the cylinder and taking the case off could result in increased tempatures. Of course we will only know if someone tries it and reports the measured temperatures.

Well this is quite useful to know. I'm going to hope, then, that I can build or buy a clear plastic version of the casing. :)