Focus your headline on the sexy work you're recruiting the engineers for, not the market your customers are in. Why mention SEO at all in your recruiting?
Oo turn it on it's head and address the fear in your recruiting efforts. Like, "how can a gig as awesome as this possibly be found in the slimy, shady underworld of search engine optimization? mwahaha."
Maybe I'm biased because I'm an SEOmoz client, but I'd imagine that once they get past the initial hurdle that your firm doesn't fit a preconception about SEO anyway. So get em over that initial bump.