I think the original definition - and the one I intended - refers to large numbers of products that sell in small quantities. A good example would be parts for 20 year old cars. Amazon is a great example of a company that does a considerable long-tail business.
While - generally - it takes more keywords to describe "long tail" products - this isn't necessarily the case. In the example above, the products are likely to have part numbers, and these single-word search terms are likely to be effective keywords.
However, IMO, the net effect in PPC advertising the the same - it's still costly for Google to open a "bin" for single-word keywords that aren't often hit.