Bruce Schneier and others[1] have done the math on brute forcing 256 bit keys: even with a perfectly efficient computer using the least amount of energy possible, you would have to deplete the entire energy content of the Sun to just iterate over a 225 bit keyspace once, let alone do anything meaningful with those keys.

Moore's Law doesn't really factor into it.

[1]http://security.stackexchange.com/a/6149

It's all down to probabilities, yes our hypothetical attacker could guess your key correctly the first time or within a few years but the chances are so tiny it approaches zero for practical purposes on practical timescales.