Don't know if anyone else had this experience...but I just happened to check Steam last night, saw that they release another video (still hadn't checked out Traptown) and decided to play it. Much to my surprise, it DID NOT download, rather started right up and played through without a hitch. The strange thing is, I didn't have my machine on all day, idle, for it to download in the background - it had been up for about 3-4 hours doing some work.
I guess that's how it's supposed to work - click and play.
Enthused by my triumph, I went to play Traptown, and had to wait for 7 minutes while my cable connection downloaded it.