In the past, having a dynamic site caused issues with most search engines. If your site has weird parameters in the URL, they were known as stop characters, and search engines would stop crawling them - in fear of getting thrown in a loop. For example, if you had a dynamic calendar system and the spider can just keep clicking next until year 3405, that is dangerous for the spider, indexer and your bandwidth and server. We have tons of articles on dynamic site topics.

As I reported yesterday at SEW Blog; Google Removes Dynamic Parameter Clause From Webmaster Guidelines. Google has removed the line that reads;

Don't use "&id=" as a parameter in your URLs, as we don't include these pages in our index.

It doesn't mean Google will index all dynamic URLs. If you have around five or more parameters, the spider still may be wary of crawling those URLs. That is why Google still recommends "rewriting dynamic URLs into user-friendly versions" as good practice.

Forum discussion at Search Engine Watch Forums & DigitalPoint Forums.