I'm working on a curses based netstat utility and just noticed that
   the m_mbufs field seems always to be zero; can someone shed some
   more light on this before I start poking through the kernel?

   kread(mbaddr, (char *)&mbstat, sizeof (mbstat))
   (gdb) print mbstat
   $1 = {m_mbufs = 0, m_clusters = 30, m_spare = 0, m_clfree = 30, m_drops = 0, 
     m_wait = 0, m_drain = 0, m_mtypes = {0, 1, 3, 0, 23, 0, 0, 0, 11, 0, 7, 
       0 <repeats 245 times>}}

A few of these alleged statistics aren't actually maintained.
Specifically:

m_mbufs
m_drops
m_wait