pfft silver isn't the best for the real shit buy a diamond heatsink! 4 times better than silver, order your today for just $10,000,000 ;P The real reason for no silver is cost, copper is the same to an extent.
As for someone's cu vs al foil crap that's be cause copper foil would be to fragile and expensive not because it wouldn't loose heat fast enough, any foil loses heat really fast due to surface area.
ok, tell me this though, if radiation is such a large factor why aren't heatsinks still black? I notice that most Al HSFs have moved to silver, even though black is the most radating colour and silver the least (leave a foil tray out in the sun, with water covered with clear food wrap and another painted black, see which gets hot
)