From the Canonical Blog.

Any new Windows 8 PC will have Secure Boot switched “ON” when it leaves the shop and will be able to boot Microsoft approved software only. However, you will most likely find that your new PC has no option for you to add your own list of approved software. So to install Linux (or any other operating system), you will need to turn Secure Boot “OFF”.

Hmmm… I wonder how many computer users will know how to do that — or understand why it might be necessary to do it. Canonical (the company behind Ubuntu) wonders about that too:

Even with the ability for users to configure Secure Boot, it will become harder for non-techie users to install, or even try, any other operating system besides the one that was loaded on the PC when you bought it. For this reason, we recommend that PCs include a User Interface to easily enable or disable Secure Boot and allow the user to chose to change their operating system.