(Roger snuck in a reply as I was writing this, so it's somewhat duplicate information...)
The entire web interface can be customized by the admin as described in the KB
. It's pretty straightforward if you just take a look at the theme directory -- theme/startpage.css for the style, theme/templates/startpage.tpl for the markup.
But no, you can't customize it on a per-user basis... it's not intended to be a hosted service, it's just meant to show off the widgets and give users a starting point for integrating them with their own web site.