The Wii Speak accessory is a microphone which allows you to chat online. Unless consumer demand increases, the plan is to slowly drop retail support of the device completely.
It's now only available at a few limited retail locations and supply will not increase unless there is sufficient demand for the product. Nintendo suggest using an alternative accessory such as the Headbanger Headset which will continue to be available and supported by upcoming games.
There was no mention of what will happen with the Wii Speak channel.