It is my strong opinion that companies place too much emphasis on SEO vs SEM. I believe the two go hand-in-hand, and deserve equal amounts of consideration and planning, though perhaps differing in cost. Because of this, I've always felt that the two should be planned and executed in tandem.
But I've run across a company called Apogee that provides paid search services. They argue that highly targeted SEM (ppc) campaigns should be run first, to filter out the most productive keywords and phrases, and this will give you the foundation on which to lay your SEO content and linking strategy. I must say, I find this to be very sound logic, and I'm beginning to think that more SEM companies should consider this strategy.