Shimano has already hinted at significant changes for the chainset in its new 2006 group set , but XTR is still king of the heap in lightweight XC terms.
The low profile, Hollowtech Duralumin arms are forged in the same piece as the simple but very effective four arm spider. This extends all the way out to the outer ring, giving increased big ring stiffness but requiring dedicated 'XTR' pattern rings.
Happily several companies such as Middleburn now make durable replacements, as while they shift beautifully, they wear very rapidly in UK conditions. The anodising on the crank arms starts to mark badly from the first ride. The reliability of the external bottom bracket bearings seems to have improved recently, although it's worth getting your frame properly 'faced' for accurate alignment. Crankarms are available in 165 to 180mm lengths to fit one and all. Price is high considering the short chainring lifespan, but it's still the lightest and easiest fitting crankset here.