The N900 does of course, like any transmitting device, send out radiation in the form of radio waves. This is totally normal and subject to very strict SAR testing and regulations before phones can go on sale - so you don't need to worry on that front.
Without going into the physics too much for the particular scenario which this thread suggests... the power level of a transmission (which is not narrowly focussed to a point) which could spontaneously cause an electrical arc in a membrane circuit - even with some radiation being conducted down a keyboard cable (which would act as an aerial) would have to be well above safe SAR levels and also totally unnecessary for mobile to mast communication. It is more likely that due to a fault or poor design that your keyboard's controller circuit is susceptible to radio interference and this is distorting real/phantom signals in the keyboard controller's circuitry and falsely acting on them (as you claim by flashing caps on and off).
You don't need to worry about it, is the long and short.