Since 1.4.4, I have experienced consistent lockups in the Nether dimension. This is usually associated with the death of a Ghast, or with the explosion of a Ghast fireball. I am playing on a remotely served game. This did not occur prior to 1.4.4, and happens with a vanilla minecraft.jar. Affects 1.4.4 and also 1.4.5. When the game crashes, the CPU of javaw.exe pegs at 25% and memory holds steady (usually around 600MB to 1GB).