A HighRanking Forums thread named Cookieless Sessions discusses one of the biggest issues with dynamically driven Web sites - the session id.
Including a Session ID in a URL, will cause the search spiders to run around like chickens without their heads. Think of it this way. I have a single page on the topic of the Big Blue Pineapple Chair. Now, I decide to serve up a different URL for each person visiting that page. Why do I do this? Sessions can be useful for tracking, up-selling, keeping track of users and so on. So by applying a unique identifier (session ID) in the URL, I can track that person throughout my site. So one spider might go to the site and get one URL, three others will get three different URLs and so on. Duplicate content disaster...
What about using cookies, storing that unique identifier on the user's computer so you can identify the user. And not worry about adding an ID to the URL itself. Well, spiders do not accept cookies for many reasons. So if you serve up a cookie, the spider won't accept it and if you require a cookie for site navigation, the spider won't navigate.
What are your options? Make sure your homepage, product pages, category pages, brand pages, and so on, work without requiring a unique identifier of some sort. I have an old but still valid article on building dynamic sites that work well with search engines, specifically Search Engine Friendly E-Commerce Catalogs.