i found what Maelstrom said pretty good --- i also do similar leg weight training although usually only in the winter (off-season for cycling).

but, also as he mentioned, your bike set-up and pedalling style are also important factors for the knees. i have had knee problems in the past, mostly from running, but also from cycling. i have really improved things by switching to Speedplay Frog pedals (with tons of float) and had a professional bike fit where riser bars and longer seatpost added. my main problem was my seat height was too low. i now sometimes experience slight signs of knee problems (usually beginning of season as i increase the miles) and i move my seat a few centimeters higher than optimal to decrease the stress on my knees (for my the sharper bend can be a problem).

as also said, higher cadence is also good for the knees -- also usually better for inceased perfomrance anyway... "mashing" feels faster but from most studies is not --- for example, Lance A. changed from a masher to a spinner and i think he's been pretty successful with the technique...

anyway, check out the bike fit as well as wokring on muscle balance. of course, also important is gradual increase in training as the tendons and joint tissue take longer to develop than the connecting muscle so in early stages of training you can damage stuff --- fighting muscle imbalance is one of the main injury prevention for knees from weight training