Just to add to the above post which already mentiond what the new features are, the DSi will also come with the DSiWare Shop, which is similar to the Wii Shop Channel where you can download specific DSi software or games. These will be region-locked, but standard DS games are still region-free.
It also comes with WPA or WPA2 wireless security support but only for DSi titles, and an upgradable firmware like what Sony's PSP has.