Nintendo Japan have announced plans to release a memory card for the Wii console that's a whole four-times larger than the existing card. Unfortunately, since the existing card tops out at a mere 512MB, that makes this new Nintendo-branded SD just 2GB.
It's been getting on for three years since Nintendo released their 512MB SD card for the Wii, which makes this new model long overdue. It'll hit Japan in mid-March, and so far the pricing is listed as "open" which likely translates to "more than you'd pay for a 2GB SD card from anyone else, sucker."
No word on availability outside of Japan, but it seems likely the other regions where the Wii is currently sold will eventually get the bigger memory chip. Of course, you could always go the unofficial route and use a 4GB non-Nintendo SD right now.