Is not scam, spam or whateva.
So, here is their official link: forums.dropbox.com/topic.php?id=52900
You have to import movies and photos from your camera/phone/whatever.
You get 500mb space for each 500mb uploaded, up to 5gb (of course, you are free to delete what you uploaded)
To get this working, you have to enable autoplay menu for devices. On windows 7, you do this on control panel-> autoplay -> enable autoplay.
On older windows versions or osx/linux... well.. start using a real OS!
Enjoy & start uploading