Your prayers have been answered, and your Memory Card 251 is about to start gathering dust... UPDATED: Bakudan brings more info from Nintendo's website.
UPDATE: Our Japanese correspondent, Bakudan Yoshinoya, just chimed in with added information from Nintendo's site.
According to Nintendo, the SD Card Adaptor will enable new ways of gaming. It's possible for friends to e-mail game data to each other. Players can also download event and game data from the Internet.
The first software to use this accessory will be Doubutsu no Mori e + (Animal Crossing e+), which will be released on June 27 in Japan. Players will be able e-mail game data and photos taken in the game through a PC.
The price has not been determined.
In a recent update to Nintendo's Japanese web site, a little surprise popped up. Nintendo is known for giving us info on accessories and add-ons for its products, then quietly shelving them, and it was feared that the same would happen to the SD Adapter. Never you fear, as Nintendo has posted a July 18th date (in Japan) for the accessory.
Also a bit surprising is a new design for the adapter, which now loads the cards from the side, rather than from the front. It will likely be much easier to remove the cards as a result.
SD Memory cards are postage-stamp sized modules frequently used in Digital Cameras and PDAs. Panasonic owns the patent on the format, and it was thought that this accessory was just a way to leverage that partnership with Panasonic/Matsushita. In other words, while the technology is cool, we didn't really expect it to see the light of day, especially after the release of the Memory Card 251. But you just never know with Nintendo.
The timing is great too, as 64MB SD cards can be had online for around $35, and 128MB cards are around $55 (we've seen them as low as $38 online). We don't yet know what the maximum size is that the GameCube will be able to address, but 64MB should work fine, since that's what Nintendo was expecting to begin with. It should be noted, however, that the picture below shows a 16MB card, which can be difficult to find.

One small caveat for importers ... it's unknown if the GameCube will be able to use the SD cards as simple memory cards, or if there are game-specific functions for the adapter. The GameCube has a limit on the number of files/blocks it can address, and a 64MB card would far eclipse that limit. It would be a good idea to wait and find out more information on the peripheral before you make an order.
We should have more info at E3. Watch for it.