If I shot some video in AVC-intra 100, can the video be downloaded or even played back for viewing on a non- AVC camera?
Say, I shoot AVC on an HPX370. Can I take that P2 card and view the video playback on an HPX500? Can I download it to a computer with the 500? Both cameras would be in the same format, i.e. 1080/60i, etc..

My inclination is no to playback of anything the camera can't record but you could use the non avc camera to transfer data in certain configurations. As long as the card mounted on the computer when hooked to a computer and you used the computer as the handler for the data you could use the camera as the P2 card holder. I wouldn't use the camera to go directly to a drive since it may not recognize the file structure of the AVC format properly.

You can export directly from the camera to a USB drive just like you could for DVCProHD material. AVCI is just the codec, the file structure is the same for P2.