Chatroom block interfers with the calendar block

When you change the month of an event calendar block, html is constantly updated with the ajax mechanism.
With a chatroom user-wide site block, ajax results shared beetwen the two blocks and calendar is replaced by the chatroom ajax message.

I reported the bug in http://drupal.org/node/155250, too.

The attached patch is a quick fix that seems to work but i don't know if it could leads to others bug.