What's in your backpack? How heavy is it? I'd guess it would be easier to get the weight off of there than the bike and you would feel the difference off your back more than by shaving a couple hundred grams off of the stem and wheels, etc. Can you switch to a bike rack? It won't change the overall weight but would likely be easier to ride with it.

Also, how much do you weigh? I know that I could drop five to ten pounds a lot easier (and cheaper) than the same weight off of my 20lb bike.

That being said, I'm all for spending money on the bike (gotta help this world economy, you know) but if your goal is lighter total weight (you, bike and gear) there are probably more effective methods you could take.