You can't predict which cards will work best (or worst) on the Sheevaplug. I think that manufacturers are focused on the camera market; ease (or difficulty) of use for computer owners isn't something that they specifically design for. And although regular storage may not be such a big deal, using an SD card as a boot device does seem to be quite challenging.
Anyway, your environment variables are listed
here; probably your simplest option is to change bootcmd:
from:
bootcmd=run bootcmd_mmc; run bootcmd_nand
to:
bootcmd=mmcinit;run bootcmd_mmc; run bootcmd_nand
To do that:
setenv bootcmd 'mmcinit;run bootcmd_mmc; run bootcmd_nand'
saveenv
reset