As someone whose primary business is training Web development teams in SEO methods, my advice is to NEVER allow session IDs in URLs. Take my word for it, Google is NOT smart enough to recognize a session ID. I also run a Web directory and I see dozens of 50-page sites each week with thousands of URLs in Google's index--all of which are banished to Supplemental Results. Anything that alters a URL can create duplicate page URLs and duplicate content penalties with search engines. Session IDs create duplicate URLs that do create problems.
Google's Webmaster Guidelines specifically cover this issue.
"Allow search bots to crawl your sites without session IDs or arguments that track their path through the site. These techniques are useful for tracking individual user behavior, but the access pattern of bots is entirely different. Using these techniques may result in incomplete indexing of your site, as bots may not be able to eliminate URLs that look different but actually point to the same page."
http://www.google.com/support/webmas...y?answer=35769
There are two workarounds for this if you must use session IDs in URLs:
1. Use PHP code to detect search engine spiders and do not initiate a session when a spider requests a page. You can easily detect a spider using $_SERVER['HTTP_USER_AGENT'].
2. Do not initiate a session until an item is placed in the shopping cart (as digital-ether mentioned). Because spiders cannot place items in a properly designed shopping cart (never use hyperlinks to place items in a cart), the session ID never displays in the URLs when spiders are present.
Hope this helps. :mrgreen:
BTW, almost every Web developer who has ever created an e-commerce site has had to deal with AOL's abundance of deficiencies. That's what happens when you do not follow Internet standards.