and from Wikipedia:
"An SD card can be used for uploading photos as well as backing up saved game data and downloaded Virtual Console and WiiWare games. To use the SD slot for transferring game saves, an update must be installed. An installation can be initiated from the Wii options menu through an Internet connection, or by inserting a game disc containing the update. Virtual Console data cannot be restored to any system except the unit of origin. An SD card can also be used to create customized in-game music from stored MP3 files, as first shown in Excite Truck, as well as music for the slideshow feature of the Photo Channel. Version 1.1 of the Photo Channel removed MP3 playback in favor of AAC support.
At the Nintendo Fall Press Conference in October 2008, Satoru Iwata announced that Wii owners would have the option to download WiiWare and Virtual Console content directly onto an SD card. The option would offer an alternative to "address the console's insufficient memory storage". The announcement stated that it would be available in Japan in the spring of 2009. Nintendo made the update available on March 25, 2009. In addition to the previously announced functionalty, it lets the player load Virtual Console and WiiWare games directly from the SD card. The update allows the use of SDHC cards, increasing the limit on SD card size from 2 GB to 32 GB."