Listing categories

I’ve hacked an advanced search Page to search multiple categories by posting ?cat=a,b,c (which does work). Is there a way of listing categories nicely given a set of category IDs? Currently I’m using wp_list_categories(‘include=’.$_GET[‘cat’].’&title_li=’) and styling the list so I get a space-separated list, but I’d like a comma seperated list like the_category(‘, ‘).
I assume that if the_category can do it, there must be a function that will work somewhere… Or am I being naive?