Basically, it's Tetris. Woo. Except for the stuff Nintendo is doing.
There are 3 things about it:
1) Choice of using either the D-Pad or stylus to move pieces.
2) Themed variants will be included, all of them NES themed. It has been confirmed that there is Super Mario, Legend of Zelda, and Metroid.
3) WiFi Connection compatability; play against other people online in Tetris, like Tetrinet, but on the DS.
Thanks to Lightning I was able to get two screenshots from Nintendo's press site:

This is most likely multiplayer LAN mode (as shown by the signal icon; a rounded color border is WFC, and the design in the screenshot is multiplayer LAN), with Mario. Wonder what the Boo item does...

This is one of the Mario modes. NES SMB3 sprites in the foreground, and a photograph of a field edge in the background. =O
Discuss.