I'm afraid I can't add any first hand experience but I'm looking forward to the replies.
My 3G plan is nearing the end of its contract and I'm also thinking of buying an unlocked 4 so I can rent a sim card and use it in Japan.
Phones: Japanese SoftBank 3G SIM Card | SoftBank Global Rental
One way to avoid unnecessary roaming is to keep your phone in airplane mode or to disable 3G when don't need it. This way, it'll prevent any sort of roaming to take place - while helping you extend battery life.
I did that with my 3G during my last 2 visits and had no problems.