Thanks for the bits ;-)
BTW why do you think I saved a Roland RD1000 ?
Do you think I can digitize the position of its 88 keys at high frequency ? :-D

From what I have read, that seems to be a pretty decent synth :) but why tear it apart ? Plus it has a MIDI port no ? Is it broken ?
Anyway scanning keys isn't very hard : usually it is done with a low-performance µC that scans the whole keyboard at 2-4kHz if I recall correctly. An Atmega8 would be overkill.

But there I'm talking about what I know : 2 crappy keyboards :]  and I don't know much of the inner working of professional ones ( but I doubt it would be completely different ! ).

On-Off is soooooo 1980's :-P I want to sample the key positions with at least 8 bits @ 1KHz...

Ah ok then xD. Well 88 leds/photo-sensors or 88 magnets/hall-sensors
and a couple of multiplexers ? I fail to see a feasible approach. 
That or some pieces of metal that make a variable capacitor, then you
measure its capacitance and deduce the position of the key ! But maybe
that's impractical too ^^

my big concern is the "crosstalk" and interferences...

Thanks for the follow! :D