I think that if Nintendo could have a firmware upgrade to allow direct play from SD cards and if it doesn't already have the ability to read SDHC cards add that in this would maybe solve the problem but SDHC cards although they will have 32gb ones out in January will still have a nice $100+ price but I also find it highly likely Nintendo will enable USB storage devices as they have earlier this year hired a company to develope software for a file system to work on a such a device I wish I could link you to this news bulletin im sure its somewhere in the forums i know it was posted before.
EDIT: found this old news article http://www.nintendic.com/news/777
EDIT 2:Later on in the interview, Fils-Aime discusses the upcoming Wii Ware, which will allow independent developers the chance to develop for the Nintendo Wii. He noted that Nintendo had been overwhelmed with requests for the $2000 USD kits, that Nintendo will not vet the downloadable games and on the issue of extra memory - a hard drive, perhaps - for Wii to hold them all on, he added, “Could there be a market for a Wii hard drive? There certainly could be.”
Found what I was looking for heres the link