[CGGorman]

Technically, it's the cam covers since the valves burried inside the head. The o-ring coil seals also develop a leak. When this happens the plugs and coils sit in a pool of oil, so I 'spose it's possible the oil wicks down the threads of the plugs onto the tip. I'd think it far more likely that when you pulled the plugs, the oil pooled up in the plug well drained into the combustion chamber and later covered plug when reinstalled.

I've not heard of ANY head gasket problems. If you DO decide to tear into the heads, make SURE to get the service manual. In addition to the usual installation procedure, there is a specific DE-torquing procedure that must be followed to prevent head warpage during tear-down. The head bolts are single-use, too. They are torqued to the point of yield and will break if you try to use them twice. Having an all-aluminum engine complicates things...