CVG recently caught up with Nintendo UK to discuss the recent WiiWare announcement. In particular, they talked about how the games will be played on your Wii. As with Virtual Console titles, the games will be playable straight from the Wii's internal memory, and can be transferred via an SD Card.
Right now we have no idea how big the games on WiiWare will be, but when asked if Nintendo has plans to release a hard drive attachment for Wii in the future, the Nintendo rep simply stated, "No."
Still, I find it hard to believe. Nintendo has to have some sort of secondary storage in the works. These games will most likely be bigger than VC titles. And with Neo-Geo games soon hitting the Wii (some of which are close to 300MB), I don't think a small SD Card will be enough anymore. If the rumors about a Wii hard drive are true, we should know for sure come E3 time.
Wii Software Q&A [CVG]