You should take a look at this Developer Zone article. It mentions what microcontroller is used in our WSN nodes as well as other introductory information.
From the linked article: "NI WSN measurement nodes are powered by a TI MSP430 microcontroller that is optimized for low-power, multiyear deployment instead of processor speed, memory, and computing power. "
Thanks David - looks like that gem was buried! As you probably know there's a whole host of variants on the TI MSP430 as described here:
It would be good to get a definite answer on the entire product range, when you also consider these microcontrollers are usually paired with a RF transceiver, such as the CC2520.