Sun hardware released after Solaris 8 no longer supports 32 bit booting; you can only run 64 bit kernels on those. This applies to all Ultra-III systems as well as the Sun Blade 100 and other UltraSPARC-IIe systems.
SPARC: In order to boot a 32 bit kernel on other 64-bit capable systems, type "boot kernel/unix" at the "ok" prompt.
x86: In order to boot a 32 bit kernel on other 64-bit capable systems, type "b kernel/unix" at the "(b)oot or ..." prompt.
Setting the "boot-file" in the EEPROM to "kernel/unix" makes it the default kernel. Alternatively, you can move away the sparcv9/unix file in which case the boot loader will fall back to the 32 bit kernel. Patch installation may restore that file, though.
Beware: when specifying "boot -s", or any boot command with flag arguments, the standard default kernel/sparcv9/unix kernel will be booted; you need to specify "boot kernel/unix -s".