Dept. of Justice Link
See especially the second to last paragraph - if JMK did anything in Thailand to a child and it can be proven, it seems to me he could face 30 years in prison in the US, according to this law.
It doesn't even seem to require that he went to Thailand for that express purpose, at least as I am reading it, anyway.
Also, from the same site, I wonder if they could charge him with this - if he has pornogrpahy on his harddrive and he brought it back with him from Thailand, could it not be construed as importing? It certainly seems to carry a much more hefty penalty than the 1 year less time served he seems likely to receive for his California charges...