"Whereas the difference between a 'professional SEO' service and the service that could be offered by a designer with a decent amount of white hat and off-site SEO knowledge isn't huge."

It depends entirely on the competence of the individual professional or not, to quote myself :-)

"The object of this guide is to show that SEO can in most cases be both simple and straightforward, and by keeping to a few basic rules you can quite successfully do your own and achieve results that are as good as if not better than many paid services."

SEO - Guide Help and Services.