Error trying to install Ruby postgres gems on OS/X

I'm trying to install the Postgres gem on OS/X but getting errors no
matter what I try..  In theory, it should be as simple as "gem install
postgres", correct?  Here's what I get:

>sudo gem install postgres
Building native extensions.  This could take a while...
ERROR:  Error installing postgres:
    ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
extconf.rb:73: command not found: pg_config --bindir

        ===========   WARNING   ===========

        You are building this extension on OS X without setting the
        ARCHFLAGS environment variable, and PostgreSQL does not appear
        to have been built as a universal binary. If you are seeing this
        message, that means that the build will probably fail.

        Try setting the environment variable ARCHFLAGS
        to '-arch i386' before building.

        For example:
        (in bash) $ export ARCHFLAGS='-arch i386'
        (in tcsh) $ setenv ARCHFLAGS '-arch i386'

        Then try building again.

        ===================================
        extconf.rb:46: command not found: pg_config --includedir
extconf.rb:53: command not found: pg_config --libdir
checking for main() in -lpq... no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
    --with-pgsql-dir
    --without-pgsql-dir
    --with-pgsql-include
    --without-pgsql-include=${pgsql-dir}/include
    --with-pgsql-lib
    --without-pgsql-lib=${pgsql-dir}/lib
    --with-pqlib
    --without-pqlib
Could not find PostgreSQL build environment (libraries & headers):
Makefile not created


Gem files will remain installed in
/Library/Ruby/Gems/1.8/gems/postgres-0.7.9.2008.01.28 for inspection.
Results logged to
/Library/Ruby/Gems/1.8/gems/postgres-0.7.9.2008.01.28/ext/gem_make.out

I have Postgres 9.0 installed at /Library/PostgreSQL/9.0.  Any ideas?

Mike