<10%, unless trends for the Core SKU sales have changed since I last looked (of course, some people that bought Cores may have also bought a HDD). edit: Of course, ~30% of 360s have RRoD'ed, and that hasn't really hurt them too bad apparently.
I always thought the 360 without the HDD was a stupid idea, and still do. I really can't imagine bothering without any sort of storage.
Oh yeah, I forgot to mention that the replacement for Dave Karakker (the major SCEA PR guy) got to finally run his mouth during GDC this week. Let's see what sort of great quotes he has in store for us this year: http://www.next-gen.biz/index.php?option=com_content&task=view&id=9264&Itemid=2 And apparently there's more to this interview that will be posted later.
