The one practical annoyment for me out of all this is that we broke
  binary compatibility in a reasonably major way.  If you use dhclient to
  obtain an IP address you now can't boot a kernel built after a certain
  date and have that work with an older userland.  For example, if someone
  boots a NetBSD 5.0 kernel (when that appears) with a NetBSD 4.0 userland
  - you lose.

  Couldn't we have just versioned the old SIOCGIFCONF that kept the
  previous behaviour under COMPAT_40 (or some other option name) and
  assigned a new ioctl command number for the fixed SIOCGIFCONF?

Yes, Christos did exactly that.  All of the problems we are talking
about are about API changes resulting from compiling on -current.

The only binary compat that I'm aware of is current from may to
september/october when the kernel was buggy, and we decided on tech-net
that it wasn't reasonable to provide binary compat within current for
bugfixes.