I installed the RC3 of Joomla 1.5. After the 'update' (made a clean install) I had several problems with missing stylesheets and broken links etc. except for the frontpage.

Meanwhile I know why. The way the basepath is used is different. You will have to change you templates! It is a minor change. You have to make sure that your templates use the correct way/path for css, javascript and whatever you need. For example:

<link rel="stylesheet" href="/<?php echo $this->baseurl ?>/templates/system/css/general.css" type="text/css" />
<link rel="stylesheet" href="/<?php echo $this->baseurl ?>/templates/<?php echo $this->template ?>/css/template.css" type="text/css" />