[juliangoldsmith]

If we were dealing with pure cosmic background radiation, or inside a Faraday cage, sure.

What I'm referring to are things like radio broadcasts, 60 Hz hum from power lines, noise put out by switching power supplies, and that sort of thing.

Just having a bias, as in your example, would be still truly random. If you knew that every tenth roll you'd get a 3, it would no longer be random. When your random number generator can be influenced by the outside world, it's no longer suitable for cryptographic use.