I'm struggling looking for references in the source tree to see what
I need to frob to get utmp to grow larger than 2592 bytes and hold
more than 64 user/tty entries (I'm trying to determine if its a 
number of users or a number of ttys issue).  The machine can still
handle more than 64 users, as they show up in a ps -aux listing.  I
have 256 tty devices created in /dev, so tty allocation is no problem.

There are roughly 131 users logged in at the moment .. and they're 
curious (some of them) as to why they've become phantoms.

rwho has had a similar problem for years.  (I think its max is 32 or 40 
users?)