Facts and maths are great, but you never know do you!
That's why I've had my vanitygen running on the satoshi wallets since 2011.
Lol @ vanitygen. I've written a much optimized version. With 30k addresses, it generates AND compares with 33 million keys per second on the CPU. Vanitygen is much slower.
2^256 is approx. 10^77, the age of our universe is 4x10^17 s approx. Even if you had a trillion machines (10^12) generating 100 million addresses per second since the big bang you would cover the 0.00000000000000000000000000000000000000001% of the 256 bit configuration space. It is more likely that a frappuccino pass through Mark Karperles by tunnel effect.
Edit: if we only consider the 160-bit space corresponding to the public key hash. The same trillion machines working since the Big Bang would only cover the 0.000000000001%, so it can be safely said that generating used keys is improbable.