We already set the username input field to a tabindex of '0'. That index is what tells the browser to allow the user to tab to the field. If it's the first input on the page, the tab key will bring the user right to it.
The problem with auto-focusing an element is that when an input is in focus (ready to be typed into) it can cause other issues with the page, like preventing scrolling. It depends on the browser how things behave when a textbox is editable, but it's something I'd recommend testing.
If you'd like to try to make the username field focused right on page load, it's pretty easy. Simply edit the /modules/mod_sclogin/tmpl/joomlaLogin_xyz.php file, where xyz is horizontal or vertical, depending on which you're using. Then, find the username input field that should look like:
<input name="username" tabindex="0" id="sclogin-username" class="input-block-level" alt="username" type="text"
Update that by putting the autofocus value in there, like:
<input autofocus name="username" tabindex="0" id="sclogin-username" class="input-block-level" alt="username" type="text"
Test that out and let us know how it goes. That's something we could add as an option to the login module so a user could choose to make the username editable on page load or not.
Thanks!
Alex