> I want to set up imake so pkg's and things built with Imake end up in /usr/pkg
> and not /usr/X11R6.. how do you go about achieving that?

Speaking for the package system:

There are two variables that are used within the package system to
govern the destination directory for installing - these are LOCALBASE,
and X11BASE. If PREFIX isn't already set, then it will be set to one of
these, depending on whether USE_X11 is defined. (With me, so far? :-))

So you could just:
1. set PREFIX in either /etc/mk.conf or your environment to /usr/pkg, or
2. set X11BASE in either /etc/mk.conf or environment to /usr/pkg

However, I can think of a good reason why you might not want
to do this.  The X11 app defaults files get installed into
${X11BASE}/lib/X11/app-defaults.  You could conceivably override this
with an XAPPLRESDIR setting in your environment whenever you run an
X client, but until everyone has a stable union FS in their kernels,
or the X consortium make XAPPLRESDIR into an XAPPLRESPATH, then we're
really stuck with it. The massochists amongst us might like to link
symbolically from the app default file to one of the same name in their
home directory, and then wonder why things just don't work so well when
they su to another user, etc.

[As a corollary to this, if you really don't like yet another directory
called /usr/pkg, and just want to put all the package stuff under
/usr/local, set LOCALBASE to /usr/local in /etc/mk.conf or your
environment. Setting in /etc/mk.conf is only supported with later
versions of /usr/share/mk/bsd.port.mk]

Regards,
Alistair