The custom firmware developers over at Magic Lantern recently released a new version of their firmware for the Canon 550D, 60D, 500D and 50D which adds an HDR function to video recording. A release for the 5Dm2 is in the works, but only out for beta testing at the moment. I feel like the footage looks a little strobe-like when there is motion, but this is still pretty incredible considering the firmware is free to anyone who wants it!
Installing Magic Lantern will work alongside your camera's standard booting firmware, but please install at your own risk! Magic Lantern has a lot of useful information about what happens to your camera over in their FAQ section that you need to read before attempting to try it out. Here is short video that explains what is technically happening with the footage being recorded, and should help you understand how this works: