I don't know about you guys, but I think more and more and more places in Japan (Osaka/Kobe at least) are accepting (read: have facilities) credit cards, and more and more "international" type ATMs are popping up. Well compared to 4 - 5 years ago definately. I any underground mall or adjacent to one of the larger banks you're bound to find a "Cirrus/Maestro" ATM.
But as far as the SAU traveller is concerned, the larger 2nd hand shops and of course SuperAB and tuning shops all take credit card. As do software shops, clothing, souvenir shops, restaurants etc, etc, etc.
So you don't have to carry heaps of cash on you... but of course save yourself the fees and carry all your cash on you