For some reason, support for the SMBFS filesystem (used to mount SMB shares, as used by Samba and Windows) is not in the NetBSD/sparc64 GENERIC kernel, even though it works and mount_smbfs(8) is part of the base system. As per this mail from Martin Husemann, to add SMBFS support, the following need to be added to the kernel configuration file:
file-system SMBFS
pseudo-device nsmb

Quite straightforward. As small as the changes are, I’ve submitted a patch for review, and if that’s OK, I’ll create a problem report with the patch attached.

Update: Hubert Feyrer has committed my patch.