well i have a bent dictaphone that had a crappy mic built in - i just straight switched it out for a minijack and it records fine from a mic or a 'line-in'... i didnt bother with a resistor as space was a bit of an issue. it works without one but if you dont want it to overdrive like hell then yeah chuck a little resistor in there. i guess you could check with a few different values first to get the 'optimum level' of volume, maybe even just throw a small pot in there
i suppose the only thing youve got to worry about is possibly frying the circuit by putting a too 'hot' or heavily amplified audio signal into the jack, basically anything that could carry a higher voltage than the circuit, but again throwing the resistor in should stop that from happening
anyway go for it, i wanna see some daisy-chained samplers!