If you're naming an operating system, best to avoid the word "Lion." You might find yourself in court, facing Apple.

The U.S. Office of Patents and Trademarks has approved the Cupertino company's trademark, giving Apple rights to the name for its "computer operating system and application development tool software." The trademark is spelled with a capital "L" followed by lowercase letters.
Apple filed the request on April 6, 2011, a few months before it filed for "OS X MOUNTAIN LION" and "MOUNTAIN LION." Neither of the later trademarks have been approved.

Not only is Apple known as a proprietary company, it doesn't hold back from litigation over its trademarks. For instance, it filed suit against Amazon last year over the term "App Store," which Apple trademarked after releasing the iPhone 3G.
This was after Apple's lengthy battle with Apple Corps, the Beatles' holding company and owner of the band's record label, over trademark issues -- a matter of dispute that was finally settled just last week.