I saw something in another post from the last few days about this- it said something about making sure you had 'shadows' turned on in the properties window you get when you double click an object - apparently the floor won't even Receive cast shadows if 'shadows' for it aren't turned on. Hope I remembered that correctly ! It's something I've wondered about too for a while. haven't tried it yet though.

" Try and be nice to people, avoid eating fat, read a good book every now and then, get some walking in, and try and live together in peace and harmony with people of all creeds and nations."
-Monty Python