- An early AP report indicated "tens of thousands."
- CBS reported 87,000 based on crowd size estimates by AirPhotosLive.com
- NBC News estimated a much larger crowd of 500,000 people
- MSNBC anchor Joe Scarborough repeatedly said "500,000"
- ABC News estimated "hundreds of thousands of people from around the country"
- The New York Post reported "an overflow crowd of 300,000 people"
Next I colored the areas which held the audience in both pics:
The area in front of the Washington Monument was a little more sparse than other areas, but still had a large amount of people in them. I didn't color the areas directly in front of the Lincoln Memorial or around it even though there were people there (you can see pics of the event at Glenn Beck's site as well as some aerial photos here -- the aerial photos have the advantage of being time-stamped). This is a back-of-the-envelope calculation.
I drew the colored areas using a vector drawing program, and so I had coordinates for the points of the colored polygons. A little math revealed that the combined area of the red, green, and blue areas bigger than the purple area, by a factor of about 5.9. If the density of the crowds were the same in the stadium that would mean about 413,000 in the crowd at the rally. In some of the areas the crowd were less dense than a stadium, but in other areas it was more dense (standing room only).
So a final estimate of about 400,000 seems reasonable, but CBS' estimate of 87,000 is borderline insane (or at least dishonest).