The Nintendo DS includes 802.11b WiFi capabilities, which will no doubt be used in the future by developers to allow for head-to-head challenges over the internet, scoreboards, and other game-related goodness.
However, this functionality coupled with the Nintendo DS microphone will also allow you to call someone, for free, using Voice-Over IP technology. This could prove useful at a location such as a foreign airport, where cell phone service will most likely be highly unfeasible, and without the long-distance charges of pay phones. Of course, this could also prove useful on school campuses or your own WiFi equipped house.
Wow sounds awesome. It gets me more excited.