I\'m using Postgres.app on OS X (10.8.3). I have modified my PATH so that the bin folder for the app is before all others.
Rammy:~          
        
Edit: Even though this answer currently has more votes than the accepted answer, the accepted answer is far simpler and cleaner.
Remove the Postgres.app binaries from the path when installing the pg gem, and instead use the postgres install built into OS X to configure the gem. The pg library will still correctly connect to the Postgres.app server later on.
Rammy:~ phrogz$ gem uninstall pg
Successfully uninstalled pg-0.15.1
# Modify PATH to remove /Applications/Postgres.app/Contents/MacOS/bin
Rammy:~ phrogz$ gem install pg
Fetching: pg-0.15.1.gem (100%)
Building native extensions.  This could take a while...
Successfully installed pg-0.15.1
1 gem installed
Rammy:~ phrogz$ ruby -v -e "require 'pg'"
ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin12.3.0]