You don’t see alot of those job listings because this isn’t a well defined field and their corporate HR wouldn’t know how to ask for that skillset. You do see a ton of postings on Craigs List for smaller companies looking for SEO help.
Most of the Fortune 500 who are pursuing SEO strategies are doing it through highly paid consultants. I have a friend who was recently employed by a mid-sized consulting company and they were doing a very healty business in SEO and SEM. Well into the millions of dollars. There biggest issue was finding people who could do the daily tactical stuff that was needed. They were pretty much resorting to hiring several people out of college to do the mundane day-to-day stuff while being managed by someone who knew their s#!t.
One important thing to take note of though is that the Fortune 500s don’t use the same tactics we do in the gambling industry, or in other mom and pop sized industries. Some of the important things we do is set up link directories on our sites, exchange links and compromise our copy to be search engine friendly. The Fortune 500 don’t generally do these things, so they need different sorts of tactics.