thanks for the suggestions, but it still doesn't work. I'll go through what I've done...
1) ordered an SD socket
2) soldered socket to *pins* on juiceware cart connector, according to this image: http://www.elinux.org/Image:Mmcschematic.jpg - I did *not* remove the juiceware connector
3) inserted 32MB MMC+ card into my Compaq Laptop, running Ubuntu Feisty, and used Gnome Partition Editor to format to FAT16
4) downloaded the sample image that used to be on the elinux.org wiki and loaded it onto the card
5) inserted the card into the socket soldered to the Juicebox and booted to diagnostic screen
6) ran mem test - result: SD card not found
7) took out the MMC+ card, and inserted a juiceware cart into the juiceware connector
8) booted normally - result: video played fine
I know that the juicebox is not fried, as juiceware still plays fine, but it won't recognize my card. As I previously mentioned, there are connections on the MMC+ card that are left floating. The limited (read:free) MMC+ spec on the company website doesn't mention any problems with floating connections, and I couldn't find anything on Google. The wires I soldered to the SD connector are about an inch, max. so I doubt that that's the problem. Would it help if I desoldered the juiceware connector? I'm reluctant to do that unless it's the only thing that will work...
any ideas? am I doing anything blatantly wrong?