I did this, upgrading from 9 spd 105 (XT in back) to 10 spd DA (XTR in back). I had a hard time finding a definitive answer as to whether I had to install new chainrings or not. The 9 speed rings worked kinda sorta, but shifting improved a lot when I installed 10 speed rings. My understanding is that the chainring spacing itself is the same, but the teeth on the 10 speed rings are set slightly off center.

And yes, chain life DOES seem kinda short.