It depends on the cable provider. With a QAM tuner you should be able to pull the HD channels from your standard cable (as you said the unencrypted ones). For instance, I have 2 Samsung 4065Fs in my house (they have QAM tuners). I have them both connected to my cable (we have extended cable but no digital boxes or anything like that). They both get the basic HD channels (CBS, ABC, NBC, 2x Fox) and I also get the big ten network HD, comcast sports HD, and a few others.
Remember, it really depends on the cable provider... but the unencrypted channels are the norm. Also, there may be a problem if you are running your cable through a lot of splitters. We have a nested set of 5 way splitters that are responsible for routing cable throughout the house. This works fine for the SD analogue and digital channels. However, we had to put both of the HD sets on the primary splitter to get HD channels in those rooms. I would presume that this is because of signal loss due to splitting up the signal so many times.