Ok, I've tried two different cards, formatted both several times, and have tried loading POI files from the feeds app (what a poorly written POS that is), with no luck whatsoever.
Likewise I've tried putting the TTS voice files on the same cards, no luck there either.
And finally, I tried putting the firmware update files on them, still with no luck.
For whatever reason, it appears as though the unit refuses to acknowledge the presence of SD cards.
I've tried formatting them on multiple computers, with 3 different card readers. In all cases, the cards show up fine with the fil