Its just what I heard "through the grapevines", at least for the official (SUN) java runtime system it seems to be true, I also read peoples comments about the technical virtues of minecraft, and these were generally that it would have been a lot better if minecraft wasn't written in Java, and that one of the reasons its often slow is because of that, or because it uses so much memory.
even running a minecraft server might be a problem:
To quote the R-PI FAQ:
Minecraft’s native libraries will not run on the ARM architecture, you will need versions of these libraries that are compiled for the ARM architecture. And even then, minecraft is very power and space hungry: 1gb of ram is a minimal if running a minecraft server. (And on top of that, you will need a good internet connection (+200kb/s upload for about 4 players.)
I still doubt minecraft itself will run well on the Raspi., if it only works with openGL, not OpenGL ES that will be the death knell, as the RPI really only supports Open GL ES, "emulating" open GL (with openGL ES) is possible in theory but not in practice.