A Search Engine Watch Forums thread asks what is the skinny on how Google views site credit links. For example, a web design company might place a sticker on the site, saying, "site designed by Company Name here."
This is a common marketing tactic, that is basically an extension of word of mouth advertising. Landscapers, roofers, air conditioning, contractors and similar companies place a sign in the front area of the lawn of homes they have worked on. Basically wanting to tell the neighbors that they have worked on this house, provided services that included X, Y and Z and a way to reach that company if they want similar services provided on their home.
You see it all the time with sites as well, a little footer link with site credits, including site designed by, site developed by, pictures by, illustrations by, sponsored by and so on.
The question is, should SEO's place an sticker that says, SEO services by?
The Search Engine Watch Forums thread suggests that SEO's should not do that. Why?
(1) The Google Webmaster Help Center says;
You should never have to link to an SEO. Avoid SEOs that talk about the power of "free-for-all" links, link popularity schemes, or submitting your site to thousands of search engines. These are typically useless exercises that don't affect your ranking in the results of the major search engines -- at least, not in a way you would likely consider to be positive.
(2) You do not want your SEO to work for your competitor, do you?
(3) Some SEO's consider it a sign of stature to never talk about or mention a client's name.
I am not sure which way to go on this. I see both sides. In my opinion, I think, that it is up to the client.
Forum discussion at Search Engine Watch Forums.