Saw this linked in a tweet by Jay Rosen

CBS News has used Digital Design & Imaging Service Inc., a Falls Church firm that employs devices such as satellite images and balloon-tethered cameras. The super-clear shots provide head counts in grids, and computers analyze areas of similar density to produce precise estimates.

Conservative commentator Glenn Beck claimed that his late August "Restoring Honor" rally in Washington attracted "between 300,000 and 500,000" participants. But Digital Design’s estimate for CBS put the crowd at 87,000. Its estimate for last weekend’s "Sanity and/or Fear" rally was 215,000 and was briefly noted on The Post’s Web site.

Post needs to get over fear of crowd estimates

I was partial to the report about how the Metro transit system had 220k more than the usual Saturday.  I thought that was a pretty good indicator of a MINIMUM attendance.  The above imaging approach came up with a pretty good indication of how accurate that may have been.