Yes; WordPress is trademarked. WP is not. WordPress's trademark policy, until late last month, stated "The abbreviation 'WP' is not covered by the WordPress trademarks and you are free to use it in any way you see fit."
"WP Fusion" is also, apparently, a trademark.
I was going the other way… if I licensed “WordPress” do I need a license for “Engine” and if not would WP Engine litigate over “WordPress Engine”?