>  But it seems this problem still exists on NetBSD 4.0 branch.
>  This is NetBSD 4.0_BETA2 on Mar 8, 2007, I think.

When I tried to reboot the machine, it printed:

syncing disks... 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 giving up
Printing vnodes for busy buffers
tag VT_UFS(1) type VDIR(2), usecount 1, writecount 0, refcount 1, flags (0<LOCKSWORK>)
        tag VT_UFS, ino 74377, on dev 255, 0 flags 0x0, effnlink 2, nlink 2
        mode 040700, owner 1216, group 125, size 512 lock type vnlock: EXCL (count 1) by pid 22360.1
tag VT_UFS(1) type VDIR(2), usecount 1, writecount 0, refcount 1, flags (0<LOCKSWORK>)
        tag VT_UFS, ino 74393, on dev 255, 0 flags 0x0, effnlink 2, nlink 2
        mode 040700, owner 1037, group 125, size 512 lock type vnlock: EXCL (count 1) by pid 26621.1
tag VT_UFS(1) type VDIR(2), usecount 1, writecount 0, refcount 1, flags (0<LOCKSWORK>)
        tag VT_UFS, ino 74391, on dev 255, 0 flags 0x0, effnlink 2, nlink 2
        mode 040700, owner 1181, group 125, size 512 lock type vnlock: EXCL (count 1) by pid 14488.1
giving up
rebooting...