SES Chicago 2005
At Search Engine Strategies Chicago 2005 (December 5-8) it was confirmed that more than 3 parameters in an URL can prevent the crawl of the major search engines for that page.
So try to limit the parameters in the URL to 3 or less. Or better yet, use an URL rewriting technique (such as ISAPI rewrite for IIS or mod_rewrite for Apache) to make search engine friendly URLs.