Richard,

AIUI, the firmware groks both the disklabel and the FFS partition. However,
by default I believe it looks for a file called "\boot" (this is from
memory, I could be wrong!) on wd0a.

To boot "/netbsd", you should enter "boot disk:\netbsd" at the ok prompt.
Note the backwards '\' character. You can also use setenv to make this the
default boot command.

Cheers, Steve