I would be very interested in the out come of this debate. I have been doing HST in the same way Scott and bilknu have - keeping 2 sets per exercise from 15s-5s. I haven't actually tried to start gaining yet (trying to do some heavy cutting 1st), but I want to make sure I'm doing this correctly. My personal interpretation of this is to increase volume if you aren't seeing or feeling anything. To grab a quote from the FAQ:
Increase volume if:
You are never sore
You are never tired
You are not growing
Maintain volume if:
You are slightly sore most of the time
You are tired enough to sleep well, but not so tired you lose motivation to train.
You are noticeably “fuller”
Decrease volume if:
You are experiencing over use pain, and strain symptoms in joints and/or muscles.
You are tired and irritable all the time, yet don’t sleep well.
Strength levels are significantly decreasing.