When I play Minecraft 1.4.2 on Ubuntu (with and without Optifine), after a while (about five minutes or so) it will crash the whole of Ubuntu, SSP and SMP (frozen interface, no responsiveness).
I would say that it's a Minecraft error because 1.3.2 worked fine. I've tried Open JDK and official Java.
The LWJGL is updated (as detailed here), which has been known in some cases to fix crashing issues. A second user with the same issue reports that it's not an overheating issue, because she keeps an eye on her system temp (Minecraft has been known to overheat laptop GPUs).
Update: Via SSH, I was able to confirm that the computer is still responsive while kde is totally locked up. Killing java resulted in nothing but some extra fuzzy lines on my screen; killing kde removed the lines, oddly enough, but produced no improvement. Rebooting the machine provided no response on my terminal. I had to hit the power switch to get it back. This may, in my case, be related to the proprietary graphics driver fglrx?