Hooray, it appears that the law and common sense has triumphed in the case of Baigent & Leigh versus The Random House Group Limited over the fiction work “The Da Vinci Code“. From the judgement:

2.1 Holy Blood Holy Grail does not have a Central Theme as contended by the Claimants: it was an artificial creation for the purposes of the litigation working back from the Da Vinci Code.

and

2.3 Even if the Central Themes were copied they are too general or of too low a level of abstraction to be capable of protection by copyright law.