The PHP version that comes bundled with OS X doesn't include PostgreSQL. You'll have to compile the extension yourself. Here are some instructions:
$ php -v
$ curl -O http://us.php.net/distributions/php-5.3.3.tar.gz
This example downloads PHP 5.3.3 but this must match your version.
$ tar -xzvf php-5.3.3.tar.gz
$ cd php-5.3.3/ext/pgsql/
$ sudo make install
extension=pgsql.so. (You may already have done this).
Update for OS X Mountain Lion Apple has removed autoconf from the newer versions of XCode so the procedure above will fail at #5. To solve that problem :
$ /usr/bin/ruby -e "$(/usr/bin/curl -fksSL https://raw.github.com/mxcl/homebrew/master/Library/Contributions/install_homebrew.rb)"
$ sudo chown -R $USER /usr/local/Cellar
$ brew update
$ brew install autoconf
That should install autoconf and allow you to install the module using the instructions above.