Just got back from two weeks in Tokyo and the answer is as follows.
I had an iPhone 3G on Optus in Australia and thought I could go and put in a Softbank 3G prepaid SIM and all woould be sweet...however I forgot about one important thing - it is locked to my provider. I was so used to my jailbroken 1st gen iPhone being unlocked that I forgot I was completely locked in with this one.
I could have pwned it and then got the 3G SIM but in the end I survived without having my phone for a few weeks and used Skype on my laptop back at the hotel each night.
So for anyone looking to do this....
It says on the SoftBank site that you can get these 3G prepaid SIM's at Lawson and 7/11 etc but I asked around and no-one had ever heard of them.
Secondly I asked at several huge SoftBank dedicated stores and no-one had ever heard of a prepaid SIM.
Finally I went to a large SoftBank store in Harajuku and found a really helpful guy who knew about the prepaid SIM and then informed me of the locking problem. He even went and got a SIM for me and tried it in my phone but we got nowhere.
So if you're going to Japan and want to use a local SIM to save on call costs and/or use 3G for email etc then you're out of luck if your phone is locked to a carrier back home - which of course makes complete sense.
So if you have a jailbroken phone you're away as long as you can find a SoftBank store that knows what they are talking about
