Friday turned out to be my lowest revenue day since AdSense was launched, but it was still within 20% of my usual weekly lows (which typically occur on Friday and Saturday).
Today (Saturday), my effective CPM are back up to normal levels, so I'm assuming the Friday slump was a fluke or was the result of fewer bids being placed on Thursday (which was the last day of July).
As for Martinibuster and Chiyo's comments, I agree with many of them, although I'd point out that the assumption that Adwords = search, AdSense = browsing isn't necessarily correct. Frequently, AdSense = a person looking at a page found via search. And even when the "content ad" clicker has been browsing instead of searching, whether he's a hot prospect or a casual looker will depend, in part, on where he saw the ad. A user who's reading luxury-cruise reviews on The Robb Report's Web site will be a hotter prospect than a person who saw an ad for Platinum Cruises in a WASHINGTON POST winter-cruise roundup article.
I agree with killroy that the AdSense program is likely to evolve, and that advertisers will almost certainly have more choices as time goes by.
It's also important to remember that different advertisers have different goals. Some are affiliate sites or small e-commerce merchants who want instant ROI, while others are large companies that are more interested in prospects than in immediate sales. Just as important, some AdWords/AdSense advertisers have little or no offline marketing experience and may be nervous about a CPC of 50 cents or a dollar, while older, more established companies may compare the cost of a clickthrough to the cost of a traditional direct-response lead and think they're getting a bargain.