-d32
-d64
Specifies whether the program is to be run in a 32-bit or 64-bit environment if available.
Currently only the Java HotSpot Server VM supports 64-bit operation, and the "-server" option is implicit with the use of -d64. This is subject to change in a future release.
If neither -d32 nor -d64 is specified, the default is to run in a 32-bit environment, except for 64-bit only systems. This is subject to change in a future release.
From:
http://java.sun.com/j2se/1.5.0/docs/tooldocs/solaris/java.html
(Looks like the same in 6.0 as well.)