I thought the the Skrulls (or at least the name) couldn't be used due to them being somehow tied into the Fantastic Four movie rights at Fox. Not sure if that is true or not. I believe there were shape-shifting, skrull-like aliens in the Ultimates books (which a lot of the movie stuff seems to come from), but I can't recall their name right now without digging out my books. Regardless, all signs point to it being some sort of alien race - hadn't heard anything about Frost giants previously, but I guess that is possible too.