Can a Japanese cellphone work with the current US SIM card?

I highly doubt it. Japan is one of the few countries in the world that does not use GSM, as you no doubt have pointed out with the 3G CDMA. Unfortunately, the only CDMA carriers here in the USA, Verizon and Sprint, will NOT activate your phone. They have a list of acceptable phones, and if you dont have one of their models, purchased from the respective stores or dealers or vendors of their companies, you will not be able to use your phone on their networks. Of course, you can always hack a solution, but I dont recommend this route.

BTW...most asian countries except Japan (though Japan does have I believe one GSM carrier) all use the GSM standards on the 900/1800/1900mhz frequencies, meaning that virtually any phone operating on at least the 1900mhz frequency will work in the USA. It just has to be unlocked, and luckily, most phone purchased overseas are unlocked because they are typically bought at full price and not that $39.99 sale price w/ two year contract (rediculous for a $150 phone), like they do in the USA.

What that modified chip is...

It's a TurboSim

Basically it's this thin sim card you put under your sim card from your carrier

When you google image it you'll get what i mean...

Basically if you insert them both into the same slot of the japanese phone. It will work.

But with t-mobile you're fine; at&t you will hardly get any coverage.

The turbo sim method unlocks the phone without putting any pins or anything in.

But the turbo sim dropps signal all the time so it could be a pain in the butt.

Basically whe you loose signal... you have to stick your simcard into a phone that has signal; once signal with the normal phone is stable

You quickly stick it back into the japanese phone.

This partially works with t-mobile; with the disconnecting
Does not work with at&t... or hardly works with at&t