For example, can I take my GM DMS cartridge and use it in a Kenwood system or vice versa?
Yes, you just need to change the firmware the box is using and to do this you can use the PhatHack DMS Tools.
http://forum.phathack.com/index.php/topic,1641.0.html
Use the 'Update firmware' option and ensure you have the option 'Reflash Box' checked.
Note: the box does NOT need to be hacked to use the tools to update the firmware...
On Ebay, I purchased a PhatNoise GM Mobile Digital Media Kit which had a 40gb DMS as part of the kit($100 for it on "buy it now"). I changed the firmware to the Kenwood Music Keg and I was in business.So it definitely works. I've not had the nerve to do the hack as I am totally computer challenged. I recently bought a 160 gb laptop hard-drive so I am trying to build up the nerve to do the hack ???