I was thinking about that myself..
But I decided to buy the official thing. It wasn't that expensive, pretty cheap actually. And not that I got it, I'm pretty sure you would need the official WiiSpeak mic. Because it had a little adapter thing on the cord. And I'm sure it's used for some sort of recognition or something of that sort..
I haven't tried it with a USB mic yet. Because I didn't own one, it was easier just to buy the WiiSpeak and atleast support Ninty at the smaller things.. xD