How to get $cat from an individual post?

Hi everyone. I’m developing a category-oriented WP 1.2 page for a friend. You can look at the progress we’ve made here: http://www.astronomyinfo.net
Anyways, I am currently using David Chait’s Similar Posts hack to return a list of posts under a given category at the side of my page. I am also using a menu.php with a switch statement based on the value of the $cat variable on each category page for a variety of different menus.
Though using the $cat variable is helpful, when you click on an individual post in a category, my menu.php switches automatically to a default value because it is not being provided with the $cat variable for the specific post.
Does anyone know a trick or function I could use to make a specific post reflect its category in numeric form?
Thanks.