Hi,I'm not a weather expert by any means, but while flying the other night from KSEA to KSFO I flew through a very thin cloud layer at 31,000 ft. Is this normal? I'm using FSX with ASX.

Sure is... what you saw is a simulation of high cirrus. They often form above 26,000 ft and are due to mixing air that lift water vapor where it becomes ice crystals. There's a short explanation in wikipedia: http://en.wikipedia.org/wiki/Cirrus_clouds

Thanks Dan for the explanation and link. I just never encountered this before or at least didn't notice it.