Alright, so here's a picture of the Zelda-themed level in the upcoming Tetris DS, in addition to the Mario-themed image already shown. Not sure what exactly constitutes a "theme" but it appears to be at least a background and a character image; maybe some music and sound effects will get added in too.

Wonder what the other themes are gonna be... obviously Metroid. Maybe some other classics like Ice Climber, Excitebike, or Kid Icarus?