From the same source as the 100-400
Canon has no imminent plans for an update to the 24-70. Canon has 5-6 lenses in queue for an update beforehand. There is a small chance you could see the lens at the same time as the 1Ds update in late 2009/early 2010.
One new lens that could be coming is a 500 f/5.6L IS. That would all but assure the end of the 400 f/5.6L. There is a market of baby boomers that are eating up higher end camera gear. Such a lens would be a sales monster according to a lot of people at Canon.
Obviously I arrived alive. Working on getting this mobile broadband USB modem working, it doesn't like Vista 64. That should get sorted so I can post a bit more often.