This is not true. It is trivial for almost anyone to distinguish 320kbps mp3 from uncompressed audio, with built-in DACs and $5 headphones, with as little as 5 minutes of training.
> Really at 320kps you are entering the realm of fantasy if you think you can hear any difference.
It depends on the encoder, the track, your equipment, and how good you are at picking out artifacts. Some people do surprisingly well in double-blind tests, though I doubt anyone can do it all the time on every sample.
That people have been doing this for many years is one of the big reasons modern encoders are so good - they've needed tonnes of careful tuning to get to this point.