Here are a few recent examples of what happens when you hire an SEO firm to “improve” your search engine rankings:

New York Times Exposes J.C. Penney Link Scheme That Causes Plummeting Rankings in Google

and this one:

Forbes Accused Of Link Spam

Both of these highlight the dangers of paying for “inbound” links to your site.  In SEO, external links to your site are one of the big ways to quickly improve your search rankings.  But the search engines are increasingly getting better at “sniffing out” these links, and penalizing sites for them.  When you hire a company to do SEO for you, check into the methods they will use, and be careful.   Keep to the slow and steady, white hat methods to build traffic.  Otherwise, you may find your site blacklisted, and all your efforts (and money) lost.