Bitcoin Specie project has been doing it, for real, with silver for quite a while now.
Gold will be added soonish.
http://coldhardca.shThe higher the volume, the lower the premium and closer to the spot alloy price.
How do I buy the silver from them ? Its not a backing as such I guess, they are offering to sell. Main problem is usually postage and customs. I'd change my mind if the Perth mint allowed depositbullion to be purchased in bitcoin and then it would be useful to be. One day I could go collect it even, ruling out delivery problems
Its available direct in quantity, or through many of the bitcoin bullion sellers. Most also offer international shipping.
Vaulting is available at the mint by custom agreement.