Google recently updated it’s webmaster guidelines to suggest that webmasters no longer re write their dynamic URLs to appear static.
This is a significant shift in SEO strategies going forward as compared to the past. The key question on every ones mind at this time is what about all my old URLs that I’ve re written? Well as“ don’t change them. If Google already has them indexed there is no need.
The difference lies in what you do going forward with new sites and work. According tot he search engine land post it only makes sense to leave static those URLs that you feel Google should know are not static as such as color choices for products in a database.
Also as “ lets not for the other search engines. While Google still has the lions share of search volume many sites still do better in Yahoo!
For more on this topic see Google Says, Don’t Rewrite Dynamic URLs To Static URLs.