After releasing my site, PHPSESSIONID's appeared in my URL's. I removed them by adding the following lines in the php.ini file in my root:
session.use_only_cookies = 1
session.use_trans_sid = 0

Somehow that did the trick for normal browsers. Textbrowsers though (and Google!) still received the PHPSESSID in the url!

I tried a lot of things (lots and lots of tips on the net regarding .htaccess and php.ini) but finally found the answer (http://www.seomoz.org/ugc/removing-phpsessid-from-an-url). The trick was adding the lines to my config.php file, being the file _config.php in de mysite directory:
ini_set('session.use_trans_sid', 0);
ini_set(‘session.use_only_cookies’, 1);

No more PHPSESSID! Next step cleaning my listings in Google...
If you're struggling like I was, now you can find the answer on the forum.