How to you really sell ideas outside of your speciality

bp. Photobooth Walkthrough from Zeus Jones on Vimeo.

How many times have you seen an idea pop up that you had. Please see the video attached. I had this one in the very early stages of digital photography with a client around 6 years ago. It was as simple as girls take photos in the outfit and print them out. (client look after cameras and photo printers).
We also had the exact same idea about three months ago for another entertainment client.
But there truth is we never made them happen.

I have always been interested in selling ideas, making them happen and understanding who does what. What if you are a PR agency and you come up with the most interesting online technology idea. But the client looks at you funny because thats the digital agency's job. So it helps lead to the question

How do you really sell thinking outside your speciality ?

There is a lot of of back and forth at the moment on what the role of strategy people are from different business types. To who is a creative, to who should control the brand. But in theory a good idea should be a good idea. But it never seems to work that way. Which sux