So I've heard a couple reports of people buying a new core system and using their HD from their banned system.

One of the speculations of banning was having out of region content on your HD, correct?
If you still had that content on your HD, it seems like that would be a cause to for MS to ban your new unmodded 360, right?

Also, I don't think if you bought a system, swapped the HD, that system got banned, and so you returned it would constitute retail fraud. You have used the system in the way that the manufacturer specified. It didn't work as it should. You have every right to return it to the retailer.
(and this is coming from someone who works for a large consumer electronics retailer)

I'm really curious about this too, and I don't understand why there's so little discussion about moving to a new box. I would think most of us with banned boxes will be moving our profiles to a new box. I'm just waiting for some kind of indication that it's safe, and won't get my new box banned aswell.

Thats probably the best explanation I've seen as to why people get banned with their old hdds.

One thing I thought of is that MS via Live could see the hdd they knew came from a banned box and then more closely scrutinize that new xbox for dvd timings/ss/etc. I mean, they may just assume that this user has modified their xbox in the past, they are likely to do it again.

But, regardless of bannings they DO want you to be on live. They probably make more money on live then on games. I think you are probably fine with the old hdd,