It seems that the device comes with a software code that can be used to download the "Wii Speak Channel," which can be used to set up chats with other similarly equipped users. The catch is that it's a one-time code, so if you end up picking up a used one, chances are you'll get one without the code or with a code that's already been used.
Around these parts, we refer to that as a "hassle." This gives you one more thing to worry about, should your Wii break. Or if you want to bring the microphone over to another Wii, say, the second Wii that you keep in your living room. OK, I'm kidding. Only crazy people have two Wiis. You may also want to be skeptical about "open box" models. Be sure to check a store's return policy extra hard if you're going to end up with anything other than a factory sealed package.
Anyways, consider yourself warned. Retailers show the Wii Speak package as selling for $29.99 starting this Sunday, right alongside Animal Crossing.