Using the widget, I can list posts relating to the current category. If I use the shortcode, I can lists posts with cat_id=x. If I want to use a template tag to do the same as the widget... I can't - as far as I can tell there isn't one, and using do_shortcode won't work, as leaving out the cat_id will bring up everything.
Of course, I might be missing something!
But here's how I got around my problem. First, determine the category id; then manually build your shortcode before 'do'ing it (you don't need all the options I'm using here):
$scategory = get_the_category(); $scat2 = $scategory->cat_ID; $shortcat="[wp_cpl_sc cat_id=" . $scat2 . " show_author=false optional_excerpt=true read_more='' show_date=false show_comments=false is_thumb=true]"; echo (do_shortcode($shortcat));
Like I said, I might have missed there being a template tag, but if there really isn't one then I hope my hack helps.