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The flip side of that is that the company who gets to use the reference may not really know what they are doing.
Success with more difficult keywords doesn't equate to being able to mix and mingle with a bunch of stiffs in suits, and the reverse is also true. Just because I've worked, or anyone else here, with 'big companies' doesn't mean we're any good.
It's the success that matters, with the little guy as much as the 800 pound gorrilllila. :)
Besides, if they tout those as references, chances are they'll charge more, and not even deliver. Get examples, and references. If those work out, you've picked a winner.
Another thing to consider is that many conpanies do not disclose client lists because it's not a very bright thing to do from a competitve standpoint. I can't begin to tell you the kind of competitve advantages I've been able to gain over the years simply by doing some digging into other seo firm's client lists.
However, most companies that don't list clients for either reason will provide examples/references to a new perspective client if the feel they are a legit. The thing to keep in mind is that if a firm can't talk about clients they work for because of some type of agency contract, then they probablly have a steady flow of regular business, so they tend to not be real motivated to jump through hoops to attract brand new clients.
That means that you won't get the info your looking off their web site, and you probably won't get it with a quick "who do you work for and what do you charge?"type phone call, but you can get it.
I'd also have to say that using who ranks well for "search engine optimization" probably isn't the best way either. In most cases, not ranking for the core industry terms doesn't have anything to do with lack of skills. It really is more about chosing the most effective ways to build a business.
I have always personally looked at it from the perspective that the time/money available for promotion is limited. I can either spend it treating myself as a client, or I can focus my efforts on doing things like landing a speaking gig at a conference of web designers who specialize in flash development.
One method is the equivalant of shooting fish in a barrel, while the other is the equivlant of being one of 500 actors who shows up at a cattle call audition. :)
It doesn't take rocket science sometimes, and tells you a lot about the firm...if you can track them, so can their competitors, which sometimes is a very, very bad thing. Other times, it can confirm what you already know: they are good at what they do.
There are some tools at Searchengineworld that help with this kind of thing, and if you look at cgi-fun.hypermart.net/ ( the site in littleman's profile) it's got a lot of tools that will help you track down who's who in the SEO biz. I used to have quite a list of stuff, but I don't do that kind of thing anymore. Gets boring, and the best work is never identifiable.