...what were you logging into ? Not your Slingbox I should think. Did it happen at watch.sling.com ? This board ?
If you saw the wrong username on anything to do with a web browser, find and click the logout link. That should clear the cookie that's remembering the wrong information. To be certain, you can delete all cookies and restart your browser. Check your browser's preferences or help documentation if you don't know how.
Hope this helps.