The Passive Voice blog, by a former IP lawyer (not currently active, and he mentions that often) has a
more detailed breakdown of the legal issues--the fact that Harper Collins tried to buy the original, and failed, is a strong point towards infringement. They didn't "just happen" to make a similar cover; they wanted *that* cover, and set about to re-create it. The author of the new cover is unhappy; she wasn't informed she was making a copy.
Currently, HC plans on using a different cover.