The d90 video, I wouldn't say is very good. Quality wise it's amazing due to good lenses. But, look at sample video with basic motion, like capturing a car going across the frame. It doesn't do action or movement very well. To be honest, I'm very inexperienced with skydiving in general, but just look at the video. Basic movement isn't captured very well, I couldn't imagine skydiving being any better.
The 5d though, might be ok. Someone recently recorded a NIN concert with a 5dmkII and it looked decent.
Here is the link to the video: http://www.vimeo.com/3388630?diggbar
Someone here with more experience should be able to help determining how the 5d would do under a skydiving test. "Motorcycle riders understand why dogs stick their head out of car windows"