One of my discoveries was that the catalogue of a certain science education company includes a lot of electronics kits. One of them is for building a switching mechanism (with various timing modes etc.) that is triggered by the light from a cellphone's screen as an incoming call is received.
They suggest using it to switch on central heating. I have a nasty feeling that it would be remarkably easy to find other uses for it, which would be considerably easier than trying to hack the internal circuitry of the phone. Tomorrow I think I may drop them a line...