return category list for use in PHP with 2.1

I’m writing a plugin that shows the pages and categories hierarchy of your blog as an animated flash movie. I do this by having the flash movie parse the page and category listings as XML.

With 2.1 I found that wp_list_cats no longer returns the list for use in PHP when I ask it to (using “wp_list_cats(‘recurse=1&children=1’)”). And I can find no option to do so using wp_list_categories.

Is there any way to get the hierarchical category listing returned as a string in PHP under 2.1?