When should you obtain written permission from people you photograph?
If a person or their property is easily identifiable in a photo or video, it is a "best practice" to have their written permission to use the image for promotional purposes.
With that said, the circumstances of taking the photos play a key role. For instance, taking a staff member's photo for their profile page on the web probably doesn't warrant a written permission because they understand why their photo is being taken. If you're at an event, and it is clear...i.e. someone has said...___ is here to take photos to promote this program"...then you're also probably on safe ground without a written permission---although this is a would be hard to prove later.
I think there's also a difference in how you use the images. If you're simply documenting online that an event occurred and here's what it looked like, then I don't think a written permission is as critical. If you then use that photo later to promote the program or CFANS in general, then I'd get a signed permission because you've taken their photo out of context and used it for another purpose.
Here's a form (.doc) you can use to obtain written permission.