a milligram of gold could be traded easily as gold leaf encased in clear plastic the size and shape of a credit card.
Shire Silver already makes these. No idea if he accepts Bitcoin though.
Sorry to wake a sleeping thread, but there's news somewhat relevant to the thread. As of last week Shire Silver does now accept bitcoin using our shopping cart! (We did accept it on an ad-hoc basis before that, but it was too clunky.)
And we are a fan of letting currencies float against each other.