Pink Monkey-
I second what others have said here about the wheels (rotating weight). They have a major impact on the feel of the ride and since they rotate (move) faster than non rotating parts, are where most of your energy is going. Cranks too, to a lesser degree.

Now while I am not a weight weenie, I do think about it, and have been working on lightening my Giant Anthem. I bought lighter wheels (ended up with Easton XC Ones but next time will have some built) and the XT crankset (much lighter than the Race Face that was on before). Between just those two, I lost over a pound! And the wheels make the bike just so much more responsive and easier for me to climb. Tires are another matter, what came with my bike were Kenda Small Block 8s. Yes very light; no very bad for anything other than hardpack. So I gained weight by switching to the F/R combo of Nevegal/WTB Weirwolf Race. Gained back almost all of what I lost in the wheels and cranks but got better loose/sand/climbing and braking.

The handlebar was upgraded to the Race Face Next SL Carbon- lighter than the stock one but not by much- but it does take some of the buzz from the grip, and the seatpost replaced with a Thompson Masterpiece (not the lightest- but the best IMO, and much lighter than the crappy boat anchor the bike came with.) And I just bought the XT shadow rear derailleur (sale at the LBS woohoo). It was much lighter than the LX! And put a lighter, more comfortable Bontrager Inform RL saddle on it.

In the end, the bike is only about a pound less than what it was when I bought it (been doing this over the last year), but it climbs better, corners better, brakes better, is more comfortable at the seat and bars, shifts better (XT Shadow shifts without that CLUNK), and I have a spare wheelset that I can mount those crappy Small Blocks on for hardpack/street riding.

So I really don't think of it as weight loss, but improving the handling and dirt worthiness of the bike, and tweaking it to fit you. That's why I say that I'm really not a weight weenie