TVersity looks to be one of the best solutions I’ve seen (so far) for streaming audio and video to your TV, from your PC, over a LAN via your Nintendo Wii. The TVersity website boasts that the software will convert video to Flash 7 on the fly so you can watch streamed video through the Wii.
TVersity have also suggested that they will be bringing out Mac and Linux version soon too. Download TVerstity.