Getting Spidered: Static vs Dynamic

Search Engine Spiders prefer Static URLs

The structure of your site is very important for search engine spiders, and search engines seem to favour static urls over dynamic urls (urls that have a ? in them). Dynamic URL's are created by server side scripts, and the portion of the url after the ? is not static.

Yaho recommends avoiding dynamically generated urls for pages that you desire to be spidered and indexed. Google can handle dynamic URLs, but they warn webmasters about using dynamic urls:

Reasons your site may not be included: Your pages are dynamically generated. We are able to index dynamically generated pages. However, because our web crawler can easily overwhelm and crash sites serving dynamic content, we limit the amount of dynamic pages we index
-- from Google Webmasters

If your site depends on dynamically generated URLs and your search engine ranking isn't quite what you feel it sould be, then you might want to consider rewritting dynamic urls into static. There are plenty of resources online that illustrate how to use Mod Rewrite to accomplish this.