The next system in the DS line after the DS and DSi has been announced in a surprise press release by Nintendo. It will be called the 3DS and is set to be released sometime within the fiscal year (which ends March, 2011). Its name is not only a reference to the fact that it is the third DS system, but also refers to its 3D capabilities which will not require special glasses. The 3DS will also be fully backwards compatible with DS and DSi games, so we can assume that it will have all of the DSi's capabilities.
As has been speculated for months, we can expect a full unveiling of this new system at E3 this June.
Thanks to Roy Karrde in our forums for bringing this up here.