i belive it has happend once and only once so far. im not sure if it was true or not but there where threads about it some time ago..
Never happened and never will happen in our lifetime or in anyone's lifetime for that matter. The sun will become a red giant and fry earth long before that.
That being said there are some terrible numbers in this thread mostly because people cant tell the difference between collision and finding out the key of a specific address.
The specific number of hashes you'd have to generate to have a 50% chance of a collision with a 160 bit (RIPEMD-160) hash (assuming that the results of RIPEND-160 are evenly distributed) are 1.42x1024
Therefore, it looks like a trillion (on the short scale) per second for a trillion (on the short scale) years (3.15X1031) would be enough after all.
As a matter of fact, if you could continuously generate and compare a trillion (on the short scale) addresses per second non-stop, you'd have a 50% chance after only 1,000,000 years.
Note that there is more involved in generating addresses than just a hash. You have to generate a private/public keypair, then generate a SHA256 hash, then generate a RIPEMD-160 hash, then compare the result to ALL the results you've generated so far. And with an ever growing number of results to compare to, it may be difficult to continuously complete the task a trillion times per second.
