Rock Band DLC for the Wii is quite exciting, but let's be clear that nobody needs to buy the recently announced $13 Rock Band 2 SD card for it to work. The Wii should support any brand of SD card up to 2GB (beyond 2GB we get into murky tech territory).
A quick search on Amazon finds that most 2GB SD cards can be purchased for under $10. For those who just can't live without the branding on this SD card (which you'll never see once it's inside the console), it'll be available this spring. Also, this PSA extends to those outrageously priced Wii-branded SD cards as well.