You can. You can do similar things with gold, paper money, and so forth. The fact that you can make it difficult to steal doesn't mean it is inherently any more theftproof than anything else.
It is pretty darn hard to make a billion dollars of gold (or paper money) virtually theft-proof, especially if you're worried that your government might decided it 'needs' some of it.
Assuming bitcoins are eventually worth billions of dollars, it will be easy to make a billion dollars of bitcoins virtually theft-proof. Heck, you could walk around with a billion dollars worth of bitoins on an encrypted memory stick in your pocket.
With a backup in a safe deposit box in Switzerland. That's MUCH better than gold or paper money.
I'm starting to think that bitcoin as a store-of-value might end up being more attractive than bitcoin as a medium-of-exchange.