Okay, what happens when you like Linux, and decide to change your Xbox Live gamertag motto to "Linux rules"? Well, Microsoft black flags you, that's what. According to xbox-scene.com, Linux and Unix prompt the following response:Your motto contains inappropriate language. Please try again.
I can confirm this as I just tried it myself. Also, according to some xbox-scene.com forum-goers, variations on Linux such as "L I N U X" or "L inux" also are banned.
I'm sure it stems from some Microsoft developers' little chuckle one night, but it's childish in the extreme. And, unfortunately, "I want to sell my firstborn to Microsoft" is taken, so I'm all out of nicknames to use on the service.