according to this kbase article usernames are "...NOT case-sensitive, and will be converted to lowercase."
I'm here to tell you that caused me quite a lot of grief today as I struggled to figure out why I was able to login and another developer was not. The story:
When creating a user and adding a domain etc I created the following (hypothetical) user:
The confirmation after creating this user told me that the setup was successful for "username" using "paSSWorD".
My documentation however for this user had him down as "userName" with the mixed case, thinking, as the kbase article described above, that it's "NOT case-sensitive" so it wouldn't matter that I left a cap in there.
Turns out I was logging in fine as I had just typed in the name all lowercase, but the developer who was going from my documentation was pasting the userName into his ftp client and couldn't login at all.
It took us a good frustrated 15 minutes to figure out what the heck was going on.
All that to say that the usernames ARE case-sensitive (and will be converted to lowercase).