It's easier to think about it in terms of the dollar/euro being weak. Sony is paid in dollars, euros, British Pounds, Australian dollars etc.
All those currencies are lower against the Yen than they used to be, so Sony (and Nintendo) is getting less money for the things they sell. Everything from games to accessories and hardware.
The reason why Sony suffers more than Nintendo is that Sony's profit margins are very low (1-5%), so this Yen issue alone is enough to get them into net losses (the currency drop is bigger than profit margins). Nintendo has 20-30% profit margins so they're much more robust.
PS: The weak dollar/euro also has some benefits for Sony such as lowering their expenses on American/European salaries. This, however, doesn't lower their expenses in Japan (or China where the Yuan is also higher lately) where a lot of their high-paid employees and real estate are. In other words, their revenue decreases faster than their expenses.
My Mario Kart Wii friend code: 2707-1866-0957
