An interesting debate has been going on over the last few years about how many of H.P. Lovecraft's most famous stories, including "The Call of Cthulhu," are now in the public domain. Wikipedia has a summary of the issue, while Chris Karr has written a detailed examination. More information is available here. The general consensus is that most of his stories are no longer protected by copyright, much to the horror of Arkham House, but that to prove this in a court of law would be extremely expensive. One thing this debate points to is a flaw in current copyright law, whereby there is continual confusion around copyrighted items passing into the public domain when their copyright period is over.