In a list of the fiscal year 2006 game line-up (April 1 of 2005 to March 31 of 2006), the new Legend of Zelda title to appear on the GCN is listed as The Legend of Zelda: Twilight Princess. You can read the full list of the 2006 games below.
GameInformer, the magazine that stated this information, seemed very sure that this title has been finalized. We'll have to wait until E3 until anyone is sure of this information, but I wouldn't be surprised if this title is real.

Many people believe that this newly rumored title has some relation with the wolf that appears at the end of the latest trailer for this upcoming title, hence the name Twilight Princess. It seems to make sense to me.

Here's the list of 2006 games that appeared in the magazine:

GAMECUBE

• Battalion Wars
• Dance Dance Revolution Mario Mix
• Donkey Konga 2
• Mario Party 7
• Pokemon XD (a full-blown RPG)
• Super Mario Strikers (a Mario soccer title)
• The Legend of Zelda: Twilight Princess (Formerly listed as The Legend of Zelda)

NINTENDO DS

• Advance Wars
• Animal Crossing
• Mario and Luigi 2
• Mario Kart DS
• Metroid Dread (a new 2D Metroid title)
• Metroid Prime Pinball
• Nintendo Tetris
• Touch Golf

GAMEBOY ADVANCE

• Donkey Kong Country 3
• Fire Emblem: The Sacred Stones
• Mario Tennis Advance (in development at Camelot)

We here at Ninformer will be sure to keep you posted on the latest updates of the Zelda title.

Sources: Cube-Europe and GameInformer Magazine

Update:

A logo was recently released for the game, which makes us almost 100% sure that this is the final title for the game. Here it is: