When upgrading or installing recent versions of the postgres ruby gem on Leopard the following error appears

	===========   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.

		===================================

The trick is to set the ARCHFLAGS variable inside the sudo command using env.

sudo env ARCHFLAGS="-arch i386" gem install postgres

Wanna know what gems you have installed without looking at all the descriptions?

gem list | egrep -v "^( |$)"

Here's a (trimmed) sample output:

$ gem list | egrep -v "^ |^$"
*** LOCAL GEMS ***
actionmailer (1.2.1, 1.2.0, 1.1.5)
actionpack (1.12.1, 1.12.0, 1.11.2)
actionwebservice (1.1.2, 1.1.1, 1.1.0, 1.0.0)
activerecord (1.14.2, 1.14.1, 1.14.0, 1.13.2)
activesupport (1.3.1, 1.3.0, 1.2.5)
BlueCloth (1.0.0)
capistrano (1.1.0)
diff-lcs (1.1.2)