Anyone who reads the JKR site will know that she has a FAQ poll where people vote for a question to be answered. The last question on there was about Secret Keepers and what happens when the Secret Keeper dies (it is still there if you want to have a read). Now in this answer she clearly states that when a Secret Keeper dies the secret dies with them ie the people that they had told still know the secret but cannot pass the secret on to anyone else.
Forward to DH page 79 (UK edition), The Ghoul in Pyjamas chapter and Mr Weasley explains why they cannot use Grimauld place any more. He says that when a Secret Keeper dies everyone who knew the secret then becomes a keeper with the ability to pass on that secret. So as Snape knew the location and was now a Secret Keeper he could tell the Death Eaters.
Sorry I know that is quite confusing but I hope you can see what I am getting at. I thought Secret Keepers would be important and when the question appeared I asked everyone to vote for that one but I am now a little disappointed. Do you think JKR deliberately gave the wrong answer to protect book 7? I could see why she might but then why post that question? I am sure this will be the first of many peoples niggles that hopefully someone else will be able to dispel with a brilliant answer
