Interactive/clickable video sounds great... in theory.

Advertisers dream of the day when their products will clickable in-stream.  As a user, I would enjoy the ability to purchase background music or a film soundtracks from within the video viewing experience.

As an experience designer, how would you layer in this interaction?  How would you notify consumers of a clickable asset without intruding on the user experience?  When does notification become interruption?  When does interruption become invasive?

I don't have an answer, but I'd love to hear your thoughts. 

Here are a few examples of what I've seen to date:

• Permission.tv puts floating target beacons over the clickable elements within a video.

• Back Channel Media will soon be testing call-to-action icons on digital TV.

• Overlay.tv (kudos to Scott for sharing) allows users to turn on and off calls to action (floating targets).

• Joost inserts semi-relevant widget overlays over video.

• HoneyShed (a client) puts calls to action for the "clickable" items on a sidebar.

Which is the best solution?  I really don't know.  But I have a feeling that we are not yet where we're going to be.