my dad's bike is a heavy cromoly commuter- tips the scales at 38 lbs with no suspension. by the time you add up the weight of a geared setup, its not THAT much heavier. yes, a hub is 1600g. but a cassette is 250+ grams for an XT (more for average), hubs are 250+ grams (deore is 450 grams!), rear D is another 250+. with just the standard stuff, you are looking at over 1000 grams on the rear wheel. add the front shifty bits, and then the shifters (another 250 grams+) and overall weight can be about even, assuming you are using mid level parts.

ignore this if you are going with high end parts- it IS lighter.