One way I can think of in which i.m.s may be interecepted is if when someone relays a renderosity url, especially in chat or other i.m. (yahoo, msn, etc) and includes the "Form.sess_id=" and the "Form.sess_key=" information - it gives access into their Renderosity account.

It would look like this basically -
http://www.renderosity.com/freestuff.ez?Form.sess_id=########&Form.sess_key=########## - or something similar and the '#' would be actual numbers.