I just upgraded one of my blogs to WordPress 3.0 and found a bug (feature?) in my theme. The category name was missing from one column on the home page. I ended up changing this line:
single_cat_title();
to this:
wp_list_categories("include=$category&title_li=&style=none");
and all is fine again. Note: The double quotes on the second command are required. If you put single quotes, the $category variable will not be replaced.