Thanks everyone. Here is my summary of what I think I read in the various replies, as well as a few more tidbits I've gleaned from reading other threads and datasheets:
Hasselblad backs can indeed be used with the 6008AF (using a relatively expensive adapter) without a cable and include EXIF information from the body (i.e. shutter speed and aperture) in the RAW file.
Sinar backs and the P20 back can be used with the 6008AF but require a cable between the body and the back, and don't include full EXIF from the body in the RAW file.
The Hy6 will be compatible with Sinar and Leaf backs and will be fully integrated with these backs, so full EXIF information will be included in the RAW files. It's still not clear whether Phase backs will be available for Hy6 (I hope they will be), or if the Hasselblad backs will be available via an adapter (again, I hope they will be). Also, if the Phase and Hasselblad backs do become available for the Hy6, it's not clear how integrated they will be, since it's not clear how open Jenoptics will be with the serial protocol between the body and back. There's been a lot of talk on this forum on whether Phase backs will be available for Hy6. It occurs to me that if Phase uses a sync cable like they did with the P20, and doesn't use the serial inferface, then it would be relatively trivial for them to make their backs work with the Hy6, even without Jenoptics support. It wouldn't be an integrated solution, but it would still work. Of course, a fully integrated solution would be preferable.
The 6008AF uses older battery technology. The Hy6 is expected to use newer battery technology which would likely provide more shots per charge.
The Hy6 gets rid of the film wind motor from the body, yielding a smaller and lighter design (I'm not sure this would save battery power however, since the motor wouldn't be used with a digital back). The Hy6 also moves the battery out of the body and into the action grip.
Someone mentioned that the backs can be rotated on the Hy6, but the 6008AF datasheet claims backs can be rotated on it as well.
One other thing I get out of the datasheets is that the action grip is removable on the 6008AF, but I seem to recall reading in another thread that the grip won't be removable on the Hy6 (I guess it can't be removable, since the battery will be in the grip on the Hy6).
If anyone else has any other points of comparison, please chime in.
Thanks again,
Hans.