In an interview on CVG Nintendo reveals some interesting details about playing classic games on the new Wii console.

One of the most interesting aspects is that your old GameCube pads work on the Nintendo Wii to play classic games on the virtual console. That means you do not need to buy the new Wii Classic Controller. Also important is that GameCube games on the Wii cannot be played with the Classic Controller, only with the GameCube one. Also an interesting tid-bit in this CVG article is that if you would import a US or Japanese Wii to Europe, it would not work with the European virtual console download server. Additionally you would void warranty, by importing. Good thing it is not necessary to import the Wii. It will be also available in Europe this year.
Buy GameCube Pads here.