It's "just" an usabillity thing, but it would be nice, if the text box for the username ("u") gets the focus after clicking the at login button. No need to "search" the box either with mouse nor with tab key, just click (@login) and type...
Furthermore I would change the order for "remember__me" checkbox and the submit button, then you don't have to go back with tab to hit the login button (I'm aware that hitting enter at the checkbox is also possible, but it's not so "natural").
Third, an mame tag for the form or something equal to anable browsers to remember past used (typed) usernames at the user text box.
<form name="login_form" action="<?php echo script()?>" accept-charset="<?php echo $lang['encoding']?>" method="post">
<span><?php echo $lang['pass']?></span>
<input type="password" name="p" class="edit" />
<label for="remember__me" class="simple">
<input type="checkbox" name="r" id="remember__me" value="1" />
<span><?php echo $lang['remember']?></span>
<input type="submit" value="<?php echo $lang['btn_login']?>" class="button" />
We could also discuss this at the dokuwiki mailinlist... Sorry, I'm not familliar with PHP and JS programming therefore it's hard for me to contribute directly. I only learned some C/C++ programming some days ago and I'm a fan of usability... ;)