For some reason, when ever I log in to a server, it works for a while, and then crashes in native code. I am usinjg java version 1.6.0_20, and the operating system is ubuntu. The log file is pretty long, but it's at http://monster860.dyndns.org/hs_err_pid29000.log
Have you verified that you have the latest available?
The crash stack doesn't point me at this explicitly, but have you tried turning off Advanced Open GL if you have it on?