Do you use a Sanyo Xacti HD1000 camera? Have you seen the green mp4 of death? Not sure why? Well, my friend Mary Matthews of Videopancakes.com also had this problem recently and she pointed me to this Apple discussion thread to help her decipher the solution. Seems that the QuickTime H264 component that came with your Xacti software worked great with your camera, but then again QuickTime gets updated by Apple every few months and that little bit of software that makes your HD files actually work gets replaced. Huge bummer if you want to, oh um, use your footage. I sat down with Mary and helped her reinstate this little component and I thought I'd make a screencast walking through it so others could fix their systems too. There are some other solutions on that Apple thread if this doesn't work, but I hope it does. Good Luck!
p.s. if you right click and save this video... you can watch it at 640x480, a bit easier to see.