The ESRB recently posted its info on the cell phone only sequel to Final Fantasy IV in the Wii portion of their database. It is titled Final Fantasy IV: The After Years and takes place 17 years after the original story. This is the ESRB's description of the game:

This is a 2D role-playing game in which players embark on an adventure involving magical crystals and a mysterious kingdom. Players traverse across a field map or through dungeons and can enter into turn-based battles along the way. Players use a menu screen to select attacks or magic spells against tiny enemies (e.g., leopards, birds, knights, fish monsters, etc.) that blink and disappear when defeated. An item called "Bacchus Wine" is occasionally used to defeat enemies in battle.

It was listed as having an E rating, and Kotaku has given out more details about the game, including character and monster abilities that change with the moon phase. They have also said that the game will have band abilities which allow multiple characters to have devastating attacks. Since this will be a 2D game, I agree with the speculation that it will be a Wiiware title, releasing later this year.