How about a "Don't ever show me this again." option? I really resent being nagged like that right up front. At that point I don't even know if the video is something I want to share since I have not seen it yet.

Or what about something less intrusive that shows up next to the video or slides down like a google video ad after I have been watching for a few minutes. At that point I'm already engaged and more likely to say, "Hey, this is really good - let me tweet it." Maybe you get more traction with the tool that way. I'm not really sure.