Rodeo - 2:25 pm on Sep 7, 2010 (gmt 0)

SEOPTI, where did you get that info from?

The message itself says that.
Here is what I have in my account:

Our system analyzed your domain, [my site], and found that you’re not opted-in to image ads on at least 50% of the pages of your site. For example, we’ve found that your 336x280 ad unit on [one of my best performing page] is only opted-in to text ads. We recommend that you opt-in to image ads for all of your AdSense ad units

.

I followed their suggestion, enabled Image ads (and text)on September 1 on 336x280 ad unit (the other two ad units are already Image enabled along with text) and saw earnings improve.
However, that could also just be coincidence since September is expected to pay more than August for me.