This topic crops up frequently in our line of work. Among the items that are often listed as important to search engines are "search engine safe" (SES) URLs. It has been pointed out that Google will index just about anything - including obscure looking URLs with cryptic parameters on them. Although this is true, we shall see that it does not exempt the developer from paying attention to the URL when he or she is thinking about search engine optimization. Let me explain.
Recently on CF-Talk Rizal Firmansyah of MasRizal & Partners (a consulting firm) gave this excellent example. He did a Google search for CFX_Excel (a noted and useful CFX tag for working with excel documents). You can see the results yourself at this link. Rizal pointed out that the top ranked link in the results pointed to a URL on his own corporate site (they apparently own this tag) and it included URL parameters. It looks like this:
Immediately underneath the top choice was a link to cftagstore that looked like this
Obviously the url with parameters managed to rank ahead of the url without parameters. Of course there are many reasons why this could be the case. Other examples on the page appeared both with and without URL parameters. Rizal's point was that his site was listed first in spite of the fact that it had a hefty URL parameter payload. Doesn't this prove that SES URLs are irrelevant? The answer to that is (in the words of the elves to Biblo) both yea and nay...
The thing to note here is that just because links with parameters are accepted and indexed does not mean you should ignore how they are constructed. Whatever is in that link is recognized as having some semantic significance to search engines. In Rizal's example the top choice was a page devoted to CFX_Excel (so the content was on point) and the URL specified some important details that the search engine probably picked up. I'm just making an educated guess, but I would wager that the word "download" and the value of productID being "cfx_excel" were both favorable indicators for the search engine. If, like a lot of ecommerce sites, the URL was "ProductID=049302", I think that it would have made some small difference. Google may still have ranked it first, but it would be one less favorable indicator.
My point is that, if you are concerned about search engines, you should pay attention to the construction of your URLs. Go ahead and use parameters, but don't be afraid to include some useful information on them. If you have a URL that looks like "...cfm?cityid=342", take the time to make it look like "...cityid=342&city=Chicago". Sure, you don't "need" anything more than the cityID on the url, but the additional information makes sense to the search engine and gives you one more arrow in your quiver.