At this time all HD set-tops use encrypted sources and these encryption schemes vary from MFG to MFG. so if you are asking if you can buy an HD set-top off the shelf and decrypt HD channels from the cable co. the answer is no, but by 2006 you may be able to when the OCAP standard is implemented.
This is CABL.com posting #131316. Tiny Link: cabl.co/mIka