The MB does flex slightly...I would not call it "twisting", more like a very slight parabolic effect. Ignoring the very good points about unleaded solder that Nichibotsu made, leaded solder has one other big advantage...it is flexible. Even when the board warps slightly, leaded solder doesn't crack.
NOTHING that goes into space uses unleaded solder. The temperatures change significantly...and unleaded solder wouldn't last a week. RoHS is a load of wank...what exactly is the health risk of having solder inside a plastic case? Surely it isn't a landfill issue...the environmentalist crowd cheered when those billions of CRT displays hit the landfills with pounds of lead in each. Then again, those morons cheered for the Prius and the super-toxic battery packs that make it more damaging to the earth than a Hummer.