> I found this one out the hard way. The problem occurs when the update results 
> in a shared library being installed with a different revision number. 
> Dependent code still references the library with the old number, yet, with 
> the "make replace" you delete the old and install the new.

This sounds like a major deficiency of pkgsrc. A library package should never
be upgraded to an incompatible version. AFAIK Debian solves this by having
library packages with revision number embedded in their name (like libpng3
or libc6). That way, packages depend exactly on the version they need, and
multiple incompatible versions of a library can be installed in parallel.

See: http://www.debian.org/doc/debian-policy/ch-sharedlibs.html
and http://www.netfort.gr.jp/~dancer/column/libpkg-guide/libpkg-guide.html

(I do not pretend to understand it all, but maybe you will find it
interesting.)

Bye	Pavel