How to estimate how many people attended an event (say, a political rally)? A student asked me today, "How do they know how many people attended a large group event, for example, the Stewart/Colbert 'Rally to Restore Sanity' in Washington D.C.?"  News outlets report estimates in the tens of thousands, but what methods are used to get those estimates, and how reliable are they?
One article apparently based their estimate on parking permits...  but what other techniques do we have?  Please note I am not talking about capture/recapture experiments or anything of the like.
I don't have any idea.  I would guess in advance that there aren't specific methods for something like this, and whatever's there is very ad hoc (such as how many parking permits were sold). Is this true? For purposes of national security - of course - it would be possible to have an analyst sit down with satellite photographs and physically bean-count the number of people there. I doubt this method is used very often. 
 A: As a follow up to this question (thanks to all for the answers), Significance magazine just published an article about this very question (it's the cover story, to boot):  "How many were there when it mattered? Estimating the sizes of crowds."  September 2011, volume 8, issue 3, pages 104-107, by Ray Watson and Paul Yip.  
It looks like they are trying to post an electronic version at the link above, but it didn't work for me just now when I tried.
A: Tim's linked article is great, though I think the company that counts people in grids is making it out to be easier than it really is.
In the local (DC) papers, I've seen quotes about Metro rider usage (except there were two other major events downtown the same day), attempts to count people at security checkpoints, grid square counting from aerial photos, quoting the numbers put on Park Service event applications, etc, none of which impress me in a large town with many things going on concurrently.
A: Mobile phone providers can count the number of phones in the area. Having an estimate of the mean number of phones/person good approximation can be calculated. This looks simple, so I assume it is in practice.
A: There are companies that specialize in counting people. For instance,
www.lynce.es$^\dagger$
(I am not affiliated nor have any interest whatsoever in such company).
They hung cameras over the groups they want to count, shoot pictures
and actually count heads. They only make small adjustments when it comes to estimate people under trees or other objects which prevent direct vision.

$\dagger$ The archived link can be found here.
A: As an alternative to WiFi mentioned by Uri, you could place Bluetooth scanner(s) in 'strategic' locations of your venue. I've attended a presentation during MPA workshop about such development in Netherlands.
A: You could estimate the people per square meter (use a few areas, of at least a few square meters each to get a good estimate) and multiply this by the size of the area.
Here is an article on this topic: How is Crowd Size estimated?
A: Here's an idea (but I am not sure this could work in practice): place a free wifi access point, and count the number of connections ( of iPhones, blackbery...). 
A: A police officer told me once that they had rules of thumb to guesstimate attendance at demonstrations (don't ask me for specifics), probably based on what Tim said. 
