Goto the following files..
Blog ->inc ->widgets ->widgets ->_coll_category_list.widget.php

Look for the following block ( app line 88 )

'option_all' => array(
					'type' => 'text',
					'label' => T_('Option "All"'),
					'defaultvalue' => T_('All'),
					'maxlength' => 100,
					'note' => T_('The "All categories" link allows to reset the filter. Leave blank if you want no such option.'),
				),

Edit T_( 'All' ), to T_( 'Main Stage'), and see what happens.
Remember to back up original files in case it stuffs something up :)

Re Hiding a Category, do you mean making it private?

Great! I got the "All" to read "Main Stage".

I formerly had a hack that made one of my categories "Main Stage" the default homepage of the blog. I now no longer want that and instead want the "All" category to be my "Main Stage".

Problem is that many outside links use the old "Main Stage" link as the primary link to our site. so I don't want to get rid of that link/page/cateogry. I just would prefer that it not be displayed in my Categories listings.

Tall order I realize but gotta give it a shot.