We're proponents of exploiting the long tail in paid search marketing. Stephan offered great insight on the long tail in natural search campaigns:
For the average medium to large sized online retailer the SEO traffic opportunity is, according to our research, 38 times greater for non-branded terms than for branded terms. Therefore an online retailer who is doing SEO right should be getting a majority of their traffic from non-branded terms. That is predicated on a very important point: capturing the Long Tail of non-branded traffic is a function of what percentage of your site's web pages are yielding search traffic.