You know, one time I asked a girl to see a movie with me...and at the time I was kinda interested in her romantically speaking...but even I myself didn't know if it was a date or not...LOL
Anyway, nothing really happened. It was all pretty platonic. We did go back to my place and talk for a few hours after the movie was over, but at that point, I already knew that I was going to be going away...so the reality sunk in my head that all we could be was friends at that point...
I don't know, if I had asked her out again and something developed, then MAYBE this would be considered our "first date"...but that didn't happen, so we're just friends now.
I think we're better friends than we would be otherwise though, because if I hadn't asked her to see that movie with me, we never would have gotten a chance to talk after the movie and get to know each other!
To the guy below (Xenophobia) who says that movies make bad dates, I disagree...they make good first dates specifically because you don't have to talk during the movie (which makes a good excuse for the awkward silence you might otherwise have), and then afterwards, it's a good ice-breaker.
But ask yourself if you want it to be a date or not? You can easily make it go either way. If you want to know if your co-worker is thinking of this as a date or not, ask yourself if this is the kind of movie he would go out of his way to see by himself...or if there is any other friends of his who might want to see the movie?
Also, if by chance he didn't name a specific movie, and just said, "do you want to see a movie with ME tomorrow" then that is definitely 99% surely a date! But he could still be hoping to make it a date, even if he did name a specific movie. You just have to figure out if he's more interested in you or the movie...