To the OP - The lightest option for FSA is the Gravity Lite's w/ ISIS. That'll end up being about 900g (weighed with their heavy single alu ring). Gaps run about 1100g, and Gravity's are 1200. Saints are 1100g (all including BB), and Diabolus are about 1150 (although might weigh a bit more in real life?). You should be able to find an ISIS bb from FSA, Truvativ and Race Face (maybe not RF?). The spec's you'd want are 83 x 128mm.
For weight, you have two options - Spend more $$ and have equal strength/performance, or spend the same amount of $$ and have less durability. In the former are things such as Ti springs, lighter stems (e-13 or Thomson), seat, post, etc.
The other option is to build up with more "all mountain/light FR" type of parts, such as smaller 2.5 tires, welter-weight tubes, all mountain rims (such as DT's 5.1, Mavic's 721 or WTB's Laserdisc Trail), thinner spokes, etc. This won't cost as much money up front, but might cost you in repair/replacement costs if things start breaking.
Overall I'd say go for as much of the first category as you can afford (Ti spring, X.0 vs X.9, etc), and then focus on light weight vs full-on DH stuff. 2.5 Maxxis DH tires with welter-weight or even XC tubes is one way to shave weight (you can actually get 2.3 DH Maxxis tires in Europe...

), as well as light weight rims/spokes. Check out the Sunday WC build for more ideas, weighed on a digital scale it is 39.2lbs (17.6k) out of the box, including pedals. Even that could be lightened w/ a Ti spring, normal (vs ODI) grips, I-Beam saddle/post, and ISIS Gravity Lite cranks. Other options would be a Ti road cassette (mmmmmm, Dura Ace 9sp CS7700 155g), e.13 Ali stem, etc.
