My experience is that it can't hurt to ask.  They might not want everybody to take pictures, but mentioning why you want photographs and having something to identify you with Wikimedia helps.   They may treat you like the press, and grant permission. I tried this twice and it worked.   

• When I was in Japan last summer (after Wikimania), I spent time in Nagasaki.  The Atomic Bomb museum there normally does not allow photography, but I asked and mentioned about Wikipedia.  I had my Wikimania badge which helped me appear legitimate with my request.  I filled out a form, they granted permission (similar to a member of the press) and gave me an armband signifying that.  The museum in Hiroshima allows anyone to photograph (but no flash).
  
• I was able to use the same approach again last September when they held public, no appointment necessary tours at the Pentagon. Normally, photography on military installations is not permitted.  I came with my camera, prepared to be turned away.  A DOD public relations representative was there.  I mentioned about getting photographs for Wikipedia and presented my Wikimania badge. He was very helpful and allowed me, along with a FOX News videographer, to stay around and photograph. 

When photographing, do keep in mind copyright issues, freedom of panorama, etc. which pertain to us.  We also need a good way of batch uploading photographs afterwards.  I had problems with the various software tools, and still have some to upload.