A lot of it (I think) comes down to the quality of the gun and the size of the magazine. Both my KWA LM4 and my Tokyo Marui HK45 have worked fine even on the coldest days, but they both have pretty large magazines (especially the LM4). The most common guns I see that have issues with the cold are single stack pistols like the WE 1911s, though the Co2 Elite Force 1911s hold up great. I have seen everything from hand warmers to using the CQB's floor heaters to remedy the colds effect on GBB magazines. I guess that sums up airsoft pretty well; improvisation.
As far as AEGs go, I have heard that batteries die quicker when it's cold though I cannot say for sure. It makes sense though, as a battery is essentially a chemical reaction taking place in the cells, and warmer temperatures typically aid chemical reactions. Without sounding too geeky, I think it would be cool to see someone test that theory out.