As a part of their newest system update, the Wii has some new functionality, including USB keyboard support in some circumstances. This means you can plug a USB keyboard into one of the slots in the back of the system, and it will work for certain things. What things, you might ask? You can use it on the Wii Shop Channel, to type in things to search for, and in certain browser cases.
What can't you use it for? Creating Miis, answering or creating poll questions, web addresses and forms on the Internet Channel, you know, most of the stuff you'd want to have a keyboard for. Still, the functionality was unannounced, which means it's likely not finished yet. When the time comes, you can bet Nintendo will let us know what it can do.
Joystiq.com: Wii update quietly adds limited USB keyboard support