Thinking aloud:

It is amazing how many SEO firms have popped up in the last few years.  Everyone seems to be claiming to deliver top 10 results – which is impossible.  In making a selection, remember one thing: a search engine builds its credibility and reputation upon the relevance of the results returned. Some of the best and brightest minds in the computing industry have helped to create the algorithms that drive the perpetual indexing of the World Wide Web, otherwise known as “crawling”.  The algorithms are commonly referred to as “spiders”… and the factors which effect change within this elaborate, complex engine are guarded like Fort Knox. This is understandable, as search engines have become so intertwined with commerce that high rankings on coveted terms can result in millions of dollars in revenue gains, or losses. See this article on some such instances where a removal from the search engine indexes of Google has caused a good deal of pain for some folks.

That said always look for indications that a firm is offering services that would represent the dishonest manipulation of the aforementioned spiders to achieve higher rankings. You may not always know the results of being pegged as a search engine deceiver – you will simply drive yourself crazy wondering why your ranking will not improve regardless of the fact that all variables which would otherwise drive higher results are aligned correctly.

BUYER BEWARE – be VERY careful in the procurement of SEO services – once a firm gets under the hood of your site and sinks their hooks into every nook and cranny of your content, it could be very difficult to undo the damage if necessary.