Google and Session IDs

It's important not to have session ids in URLs that are crawled by search engines, otherwise the URL for each page changes every time a search engine visits.

It's interesting to note that there 30,500,000 pages indexed in Google with ?phpsessid= in the url:

http://www.google.com/search?q=inurl%3A%22%3Fphpsessid%3D%22

Google's Webmaster Guidelines say the following about session IDs:

"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."