George and others interested-
Found a definitve explanation:
http://www.hometheatermag.com/hookmeup/406hook/

specific to George's ?:
"The HD-DVD format specifications mandate the use of Dolby TrueHD and Dolby Digital Plus. BD specs require the original Dolby Digital and allow Dolby TrueHD and Dolby Digital Plus as an option."
"BD and HD-DVD both mandate the original DTS we've come to know and love on standard-definition DVDs. The other elements of the DTS-HD package are optional but tantalizing."

ken