Well, we know that at least Draco Malfoy, Bellatrix, Voldemort, Snape and Dumbledore know it. I bet my money though, that every Auror and the more intelligent Death Eaters know it.
Moody for example knew it for sure.
I always thought that Dumbledore wanted to use the Occlumency lessons as a way for Snape to bond a bit more with Harry. After all, he knew he would die and to set up Snape as a more trustworthy person would be a good move.
There is of course also the factor, that with Snape Harry had a more pressing need to learn.

Mh, yeah, I went with that explanation for a long time too. But I'm also not sure it really his canon and not fanon ...

Exactly. :p So many new ideas!