I'm going to Japan in April, and I checked into this a bit. They do have CDMA networks there, but it looks like they're on different frequencies. I think you can get voice, but no 3G with the verizon droid, but I can't recall exactly.
I didn't pay that much attention to it since I plan on just using Wifi and sipdroid while I'm there.
And here's the next thing to note, apparently it's hard to find wifi in Japan. Hotels almost never have it. Actually, most hotels don't even have internet in the room, since the 3G network is so good and cheap there, everyone has 3G phones and 3G cards for their laptops.
You can get wifi in McDonalds I think, and a couple other places, but it might be tough just roaming around looking for an open wifi signal.
Some hotels will have ethernet internet in a room, so I think I'm gonna book one of those and get a travel router.