I removed the open-java whatever that Ubuntu prefers to use. I installed
sun-java6-jre. Then I ran
sudo update-alternatives --config java to ensure I was using Sun's Java. Restarted my browsers, then tried to start the game from: http://www.minecraft.net/play.jsp
It pops a prompt to make sure I really want to allow direct interaction between minecraft.net and my computer, I agree. The in browser display mumbles something about loading a library, then darkness. The page doesn't crash,
ps shows the java plugin process, all seems well except the game doesn't load.
I see the same behaviour on Chromium 11.0.662.0 (73963) as Firefox 4 RC1.