Add a sprintf style argument to wp_list_categories() for title tag.

[mfields]

By default, wp_list_categories() will use the value of $term->description for the title tag of every link that it generates. If the argument "use_desc_for_title" is set to false, it will produce a string like: "View all posts filed under $term->name". With the inclusion of custom post_types into core, it would be wonderful to have more control over this title. The best solution that I can think of is to add a sprintf style argument which will allow themes and plugins to provide a custom format. Something like:

'title_format' => 'View all tacos posted under %s.'

Seems fine to me.