Phil,
NASA cite that as the reason for using stone age (relatively) IT in the shuttle/sattelites, as modern processors are constructed on such a small scale that radiation/cosmic rays etc is more likely to interfere with it's normal operation, so it doesn't seem an unreasonable concept.... suspect it's all a question of saving money though! Bet that your old 486 you put in the recycling section of the tip ends up on the 'phoon!
Correct. Currently, radiation hardened technology is being built on 130nm processes that are 5-years behind the cutting edge 45nm technologies. That said, there are some technologies out there that allow newer processes. Also, it is possible to design around some of the problems with error checking or even do the same work on, say, five different processors and choose the majority result (in fact, I think the Typhoon uses this approach).
Anyhow, the reasons for the old technology in military systems is two-fold. Firstly it takes a long time to design safety critical systems (both hardware and software). This isn't unique to the military space as the design and qualification process for a car ABS system is around five-years. Secondly, when choosing a supplier you want to make sure you can still get parts in 25-years. Intel will guarantee that certain products will be supported for a very-long period of time.
Anyhow, military technology is always a long way behind civilian tech. Indeed, a high end cell phone has a similar level of processing power as the Typhoon, but then again, if your phone crashes it won't cause anything like the same problems!