Crash upon entering the Nether 1.5

Possibly just Java being unreasonable, but I get a horrendous crash whenever I try to log into a server where my character is in the Nether. I sometimes see the World for a few seconds before crashing, but most of the time it never leaves the log-in screen and just tells me it's stopped working. Or even entering the Nether on a server i'm already logged on to.