Yup!
Tonight I was getting caught up on How I Met Your Mother (last night's episode) and I thought a tall guy in one of the scenes resembled Conan O'Brien. At first I figured it must have been an extra that resembled him, if you're a fan of the show, hisdoppelganger, if you will.
Then I went back and watched it again and realized, "That IS Conan O'Brien."
Here's a screen grab of his appearance. I've added a few very subtle arrows to point in Conan's direction... :)

I'm surprised I even noticed his cameo, because he was only on camera for about five seconds. I guess an 8-foot-tall (slight exaggeration) red haired goofy lookin' dude sticks out. Heh.
Here's video, see if you can easily spot Sir Coco.
After doing some research I learned Conan O'Brien promised Neil Patrick Harris back in June that he'd appear on the show.
So yup, that sure was Conan O'Brien.
Badabing Badaboom, case closed.