My access log gets filled by entries like this, which occur every 5 seconds.

xx.xx.xx.xx - - [18/Aug/2010:13:31:00 -0400] "GET /flash/XML.php?module=im&action=updateInvite&recipient=3&_t=1282152666655 HTTP/1.1" 200 70 "http://mysite.com/" "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.8) Gecko/20100722 Firefox/3.6.8"

I disabled the IM module to see if things get any better