Even the IEEE can't get it right, but that's a capital X, people: It's just one of those things that makes the proofreader in me go nuts. Is the 802.1X authentication standard an uppercase X or lowercase x? I thought for the longest time it was a small letter. The IEEE often refers to it that way, and that's in keeping with the 802.11a, b, e, etc., Task Groups that we know so well.
Matthew Gast maintains that the answer is clear: 802.1X is a standalone standard. These types of standards receive capital letters from the IEEE. Supplements use lowercase letters.
Why does this cause my teeth to grind? Because you find journalists referring generically to 802.11 devices as 802.1x or 802.11x, both of which are clever but confusing ways to talk about them. I always say "the 802.11 family of standards," or "standards from the 802.11 working group at the IEEE." This clearly defines what I mean. "x" is so often used as a generic that as we hear more about using 802.1X in public places, we're better off making it an accurate capital to distinguish its use.