Back in 2005, as part of the testing for the new coupled ocean models now running, some testing was done with 64 bits. These models didn\'t last long before crashing, and never made it out of Alpha testing.
The problem was the coupling between the atmosphere part and the ocean part, which couldn\'t be made stable in 64 bits.

And one of the programmers said in the restricted access area used by the admins here, that there wasn\'t much improvement in speed. As I recall, it didn\'t reach double figures. This is apparently because the models rely heavily on floating point calcs, and there wasn\'t much change in the fpu between the 32 bit and the 64 bit processors.