I think I did something like this...
I used the Log In Widget from http://www.silverstripe.org/Log-in/
Then you need to add something like this to end of mysite/code/Page.php
---------------
function MyLogin() {
$widget = new LoginWidget();
return $widget->renderWith("WidgetHolder");
}
---------------
Then you need to call this somewhere in your theme. It could be theme/blackcandy/templates/Pages.ss
---------------
<div id="BgContainer">
<div id="Container">
...
...
<div id="Sidebar">
$MyLogin
</div>
...
...
---------------
I also think you need to call http://yoursite.com/dev/build