While Nintendo has been pretty good so far with the Game Boys of yore and the DS / DS Lite at keeping backwards compatibility pretty much at a maximum, the DSi's added functionality and processing power is going to test that a bit. Nintendo has confirmed that there will be two new types of DS software, "DSi Cards" and "DSi Enhanced Cards." The former will only work with the DSi (and with the current DS install base, we're guessing they won't be incredibly common, but the DSi's off to a strong start) and the latter will offer expanded functionality on the DSi (like mapping a photo onto a character) but will still downgrade gracefully to regular DS handhelds. This could certainly cause some confusion, but hopefully anything DSi specific will be very obviously DSi-centric, like a camera app or something to automatically spend thousands of Nintendo Points on classic titles in a flailing attempt to relive your childhood.