So Here's a silly idea:
You want to verify a photo was taken a a certain time. You could go get a copy of todays paper, or you could go to phototimestamp.org (just registered it)
When you go there, a QR code (similar to this) is generated.
Then just take a picture of whatever, with the QR code in the background (changed every 15 minutes).
Then the other user can submit the image, and the time that it was claimed to be taken at, and the website will spit back true or false.