I think it's going to end up like a lot of other creative and consulting specialties. I look at business writers (copywriters and technical writers) for a model.
You'll have some experts who go it alone, totally freelance, getting their own direct clients. Some of them would be well-known, like copywriter Bob Bly or our own Karon; others might be "local experts" who have simply built up a stable of satisfied customers and referrals in their region or city.
You'll have some people who prefer not to deal with the client acquisition hassles, who will subcontract for others. Those "others" may offer specialty SEO services or may subcontract the SEO as part of a comprehensive package of services. (There are companies today that specialize in providing technical writers and general consulting firms that offer technical writing as one of their services. Both can and do hire independent contractors to actually provide the services to their clients.)
You'll have some people who will become the employees of consulting firms. As above, these firms may be specialty SEO firms or may be more general in scope. The main difference between these people and the subcontractors above is that these will be actual employees who only work for the one firm, while the subcontractors are free to work with any number of firms at one time.
I worked for my last four years in the NYC metro area as a tech writer with firms of both types. You had more flexibility with the first type, but better benefits with the second.
Then you'll have true in-house people who are employed by a company to perform SEO-type services specifically for that company. That would be the type of position that Elizabeth now holds.
There are plusses and minuses to each approach, both for the service provider and the service buyer, and the different models are appropriate for different situations. For the service provider, it's a matter of how comfortable they are with self-marketing and sales, and how much time they want to spend on business-building and administration versus actually doing SEO work. For the service buyer, it's a matter of how much they want to spend, whether they anticipate ongoing work over a very long time frame, whether they have enough work to make it worth hiring a full-time person, how much they typically spend on benefits per employee, and what the labor market is like in their area.
I don't see any reason why SEO couldn't be like business writing, or consulting, or any number of other similar knowledge-based professions, in which all business models are accomodated.