I remember I was discussing with David a month or two ago and saying I would buy a Canon EF 70-200 F4 lens if it was available in an IS version and replace my 70-200 F/2.8 non IS version with it.
Well it was just announced:
http://www.dpreview.com/news/0608/06082414canon70-200f4lens.aspBut the motivation to give up a F/2.8 lens for an F/4 lens was to get the IS function and reduce size and weight as well.
The problem I see is that the F/4 IS lens that was just announced will sell for nearly the same price as the F/2.8 IS version. That and the size and weight is nearly the same as well.
So if the F/4 version only provides a small reduction in size and weight over the F/2.8 version and the price difference is marginal, then I don't really see enough of a benefit to buy the lesser F/4 version instead.
That is just me, and if others have a different view on it I would be interested to hear it. :)