Hey Birdcat!!!!
 Yes that is an issue. I thought of this. When you join the system you are assigned a number, all footage you upload must contain that number in the footage perhaps at the beginning or the end. Or spoken. This may verify you shot the footage. If you shot it prior to joining or cannot include it then you may have to sign some electronic agreement signifying you did shoot the footage and exempt FF from any liability. Probably need to do that anyway.
FF would be just the conduit to move footage, you as the videographer are responsibility for it. If you sign something saying you shot it and you didn't, then the responsibility falls on you for the infringement.
Frankly I think this would be rare but some folks may try to pass off footage they don't own. You could also have something they have to submit to verify that any public domain footage is indeed in the public domain.
Now we live in an electronic world and things like this site once thought only as fantasy can easily be created. Joel is a pioneer here, and the copyright courts will need to adjust to the new and changing ways artistic materials can be shared and sold.
If I rent a truck and go steal a famous painting I am responsible for the theft, not the truck. As the receptacle for video art Joel is providing us with a service, WE are responsible for using it correctly.
I see this growing into a major virtual warehouse for stock footage. But we will probably have to help police it ourselves.
J,
