Future of advertising agencies - paid for ideas or campaigns?

I spoke recently at an event for advertising agencies: Big issue for the future is how people will get paid?

In the past the ideas came free and corporations paid a commission on all spending on the actual campaign. But in a digital world many campaigns have little or no direct running costs - see for example viral marketing of a funny short video clip which carries a corporate message. That means paying for ideas, which is also tough because it is hard to value an idea and the best may emerge in meetings with clients so that it is impossible to prove who contributed what to the process.

Tough times ahead as the digital world forces a complete rethink about the whole relationship.