I just updated from UTW to WP2.3-Tagging with SimpleTags but there seems to be no way to get what UTW_ShowCurrentTagSet() did.
Unfortunatelly the wp_title() or (the same) single_tag_title() only outputs one tag.
Like URL /tag/foo/ gives the output foo.
But URL /tag/foo+bar/ also gives only foo.
So right now there seems to be no propper replacement for UTW_ShowCurrentTagSet
The problem is also described here http://wordpress.org/support/topic/136356#post-686488 and here http://www.lifespy.com/2007/how-to-switch-from-ultimate-tag-warrior-to-wordpress-23-tagging/#comment-118283