http://www.macrumors.com/2008/10/31/whispers-suggest-a-mid-november-imac-and-mac-mini-updates/
http://www.fudzilla.com/index.php?option=com_content&task=view&id=10224&Itemid=1
Fudzilla claims that they've been hearing whispers that iMac and Mac mini updates could be coming as soon as November 10th. They are not particularly confident about the date which would fall on a Monday.
While we don't place too much confidence in Fudzilla as a rumor source, the information is consistent with circulating expectations of pre-holiday iMac updates. Apple typically issues updates on Tuesdays, but has had an occasional Monday release including the last iMac update on Monday, April 28th.
http://www.fudzilla.com/index.php?option=com_content&task=view&id=10224&Itemid=1