Cheers guys. Great site Nick, btw. I was a bit apprehensive to work with a stripped down system (ie MCU plus basics) but I think I might give it a crack now.
As far as the power switching goes, I've been looking into voltage regulators, and seems there are quite a few with an enable pin on them, many of which quote ~1uA current draw when in the shutdown state, which looks promising, like the LT3060 (LT3060 Datasheet and Product Info | Analog Devices). I figure I have a couple of these for the 3.3V and 5V components and use the MCU digital pins to shut them off before moving into a sleep mode.
Can the watchdog timer be programmed for long intervals? (ie every hour?) I noticed the RTC DS3234 has a quiescent current of the order of 120uA, which is pretty large considering I might be able to get the MCU and peripherals down to tens of uA. Does anyone know of any ultra low power RTCs?
Thanks again for all your helpful replies!