yes SD has a hardware level lock on it that you can, in theory, lock the data in such a way as to not have access to it (in any form) unless you provide the sd card itself with the key.
Although this "feature" was a bit concerning when it came out, it seems to have rarely gotten used. Maybe by some government types. I don't even know if Mac OS X or any other OS such as WIndows has the capability to access a secured SD card. I've never encountered one nor heard of anyone else that has.
Although I DO find some other interesting points. First of which is that my PowerShot S3IS camera can boot off alternate firmware to gain access to additional features. This boot process, curiously enough, requires the SD card's "Locked" slider to be in the LOCKED position to boot. And while booted off said card, pictures can be taken and stored normally. (this Lock slider is unrelated to being "Locked" / "Secured" by the Secure Digital card's namesake password protection)
Naturally that gets types such as myself curious, and I took apart a 32mb (thanks canon for that bundled card!) to see about the lock, and as expected, it's just a plastic slide that has no electrical connection. The locking function is entirely regulated by the device you plug it into. (I've encountered people on more than one occasion that insisted that the card could not be accidentally erased if it was in the Locked position) Reminds me of client-side security. And of the Write Protect notch/slider on 5.25" and 3.5" discs.
Makes me wonder what sort of security it really has when "Secured". I bet it's just another doorman like the Lock tab, and that your data isn't encrypted in the least, it's just a little harder to get at.
