The hiss has to do with the combination of HTC's headphone amp and your headphones' impedance. Some expensive headphones have too low of impedance for the phone (as is my case) so even they will hiss.
There is a fix for this, in addition to getting different, higher impedance headphones. I discussed this in my previous post
here.
I know this thread is a little old, but after getting my FOURTH Droid Incredible 2, I am convinced that this is something that is present in all models, but certain people either don't hear it or are not using headphones that accentuate it.
I only use in-ear headphones, and it is extremely noticable. Sure, I could probably use another kind of headphone, but I don't think I should have to do that to make my a $600 piece of electronics work as expected.
I have decided to just stick with the phone and suck it up until a new phone comes out (I'm out of contract, so I can just on any new phone that interests me). From looking around online, there are posts similar to this about every HTC phone, so it appears to be a problem with their phones in general.
Quite sad, as they make a good product otherwise.