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.
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
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.