SendKeys on selenium webdriver once in a while sends text in upper case in Internet Explorer 10
Wouldn't be surprised if it was a bug. I'd suggest changing your IE version from 64 bit -> 32 bit or vice versa to test.
Also if you're on 10.0.11 (November 12, 2013) try an older build. For example when I use an older build of chrome sometimes it resolves these kind of issues until the selenium community sends a fix for that browser version.
Lastly, if you're using random generated usernames. Try using a downcase / lower method on the username before sending it to keys. Something like
random_username = "John"lowercase_username = random_username.lower()then inject lowercase_username into your selenium code
this.driver.FindElement(By.Id("uid")).SendKeys(lowercase_username)