From what I read on LyngSat, Dish uses 8PSK on transponders carrying HD only, whereas Voom uses 8PSK for everything. DISH uses QPSK on their transponders carrying SD.
But yes, the big deal is the encryption: VOOM uses TWO standards: Motorola's DigiCipher II and the NDS Videoguard system. DigiCipher II uses no access cards, whereas NDS Videoguard uses the card in the front slot of your STB.
DISH uses open-standard DVB compression, but with a different card-based system called NagraVision.
Interestingly, Motorola does make a set top box that supports DVB-complaint streams with any type of card-based encryption...and the box uses the same cabinet as our Voom boxes:
http://broadband.motorola.com/catalog/productdetail.asp?ProductID=383
But honestly, I think it's cheaper for DISH to just discontinue the Voom data stream on those 11 transponders and just start broadcasting DISH content. SOOO much simpler and cheaper for them!