Controlling my single category sidebar

Here’s my problem I have a category page (Category) which has a listing of all pages contained in that category in the sidebar.

But when I click on an entry the list disappears.

I would like to have the listing of categories in the sidebar change per post depending on what category that post is in.

So, if you visit a page in the about category, a listing of about’s entries appears in the sidebar.

(I’m assuming no multiple categories will be selected)

I assume I have to write an <?php if (belongs to this category()) { ?> tag for each category name in my sidebar? is there anyway to get the category type from the post then relay that to the sidebar?

Am I on the wrong trail?

Help, my template is taunting me!