Or said in another way, does packing lighter on a trip actually make a dent in one's co2 emissions?
According to The carbon independent site,
CO2 emissions from aviation fuel are 3.15 grams per gram of fuel  , which gives CO2 emissions from a Boeing 737-400 of 115 g per passenger km.
What it does not talk about, however, is the weight of the passengers and their luggage. Any write-ups on that?
What are the average per-passenger-kilogram carbon emissions of common commercial passenger aircraft per kilometer?