Check out this memory ranking table. The first four are using an unknown type of memory called Adata PC4000. Like you can see the first one was capable to get DDR628 with that stick! The second, third and fourth in the table were also capable to get past DDR600.
My Japanese ain't well but if I'm correct the second one was running at DDR606 at timings of 3-4-4-8.
Here in Europe and in the USA this memory is not so popular and I think hard to get, but at the moment in Japan it appears to be very popular.
has some pictures about it. An Adata PC4000 256MB memory module rated at Cas 3 appears to cost 7800 Yen , this is about $66. Not really that much for modules which seem to be able to go to +DDR600 and beyond! Hopefully we will see them soon here in Europe and in the USA.