This is a strange one so I'll go through how to reproduce this step by step. As far as I have seen this happens in FireFox 3.6.x on both Windows and Mac. First you must be logged into the forum and need to have the rich text editor disabled. Then navigate to any thread and refresh the page with F5. After the refresh somehow the quick reply field becomes pre-populated with "Update your status...".
Certainly a minor bug, but worth reporting still I thought.
Hmm actually I'm not seeing this happen here on XF, the software I am seeing it on is an RC3 install with several add-ons installed. Likely it has something to do with the the official forums being on a build other than RC3 or a result of one of the add-ons. Thanks.