CL wrote:
> I have also been able to set up temporary mail / data accounts through
> Softbank (but I am a Softbank keitai and BB subscriber) which fall on
> the high side of Japanese rates, but are still cheaper than pay-as-you
> go plans.

The flat-rate data bonus pack looks interesting :
http://mb.softbank.jp/en/products/data_com/price_bonus.html

And the japanese version confirms it still available at about the same 
prices :
http://mb.softbank.jp/mb/data_com/price_plan/bonus_pack/#price
The standard data rate is another good choice :
http://mb.softbank.jp/mb/data_com/price_plan/data/
although the english description is a lot more expensive, so I'm worried 
if I missed a few things in the japanese page :
http://mb.softbank.jp/en/products/data_com/price_value.html