Microsoft Reportedly Requires "Signature PCs" To Be Locked To Only Running Windows
Ryan Farmer wrote in explaining that his Yoga 900 ISK2 UltraBook hasn't been able to see Linux installed over a proprietary RAID mode that's locked by the UEFI/BIOS of this ultrabook: Linux can't see the SSD. When contacting Lenovo, he was told by a Lenovo representative, "This system has a Signature Edition of Windows 10 Home installed. It is locked per our agreement with Microsoft."
Microsoft Signature Edition PCs are those that come without any pre-installed bloatware (per Microsoft's own description), comes protected with Windows Defender by default, ensures solid keyboard quality, meets Microsoft touchpad sensitivity standards, and meets other Windows hardware certification requirements. And according to this Lenovo representative, an unadvertised requirement is that it be locked to running Windows 10.
There is a wide assortment of Microsoft Signature Edition PCs from different vendors, including laptops, tablets, and desktops. Those interested in seeing the Signature Edition hardware can do so via the MicrosoftStore.com.
Some more bits of information can be found via this Ubuntu Forums thread. There's also been much discussion on the Lenovo Forums about other customers running into problems of being unable to install Ubuntu and other operating systems.
Update: It looks like the initial information from Lenovo was incorrect that it's likely not an intentional Linux block.