There would need to be a downloaded application from Apple's ApStore, which would link to a card reader that fits into the charging/sync port at the bottom of the Touch. So far, nothing like that's been developed that we're aware of, and we're not sure Apple would even allow it because it would be an insecure route to transfer data/programs into the Touch or iPhone. Everything (so far)has to be ported to the device through the ApStore.
An SD card reader would be very useful for dongle-based applications or data readers, especially for PocketPC users.