From their page about username policies:
Wikipedia usernames are case sensitive, but the first letter is always automatically capitalized.
But I've also noticed that, even though I always login by typing "matt" (not actually my username, of course), I can still login, even though they change it to "Matt". So is it really case-sensitive? And why do they do this? I've never seen any other site capitalize the first letter of a username before.
Best Answer
Usernames are a subset of page titles, and must follow the rules for page titles. Mediawiki, the software behind Wikipedia, treats by default all article titles as beginning with a capital letter (unless the first character is not a letter).
Note that
$wgCapitalLinkscan be used to disable auto-capitalization for page titles. According to this discussion, usernames are always capitalized for compatibility though. (might have changed since then?)