Wow I wasn't expecting quite such a big response to my initial question!
I actually dug up this info document from CASA (Civil Aviation Safety Authority, for non-Aussies), a little out of date now but still quite interesting.
http://www.casa.gov.au/newrules/part.../ac091-050.pdf
Apparently the degree of interference also has a fair bit to do with the location of the device inside the plane, the variability of the signal and what the plane's electronic systems are doing at the time.
Electronic watches have been tested and found to be safe, but others including electronic games have been shown to interfere with navigation.
Methinks I'll be good and turn my ereader off, annoying as it is.