Well, with these pictures we've finally got a good look at the underbody of the HX, and it is definitely built off a shortened H3 frame. High-res versions can be found here
http://www.flickr.com/photos/richard...n/photostream/
You can see the H3's frame rails, crossmembers and shields, along with the inner wheelwell plastic covers. It also still still sports the leafspring setup of the H3, along with the torsion bar front.
The HX being built off the H3 platform would be a great way of keeping HUMMER and the Shreveport/Africa plants running. It's obviously a perfect fit.