As I said before, if you have DoD (if you're not familiar, Department of Defence) level stuff on your SD card, then go ahead an encrypt it.
Else, it's way too much trouble for the general user than it's worth the added "security".
If your phone is stolen, do you really thing the theif will want your files? Wiped device, formated SD, that's what happens.
The security is on both hardware and OS level. There are posts on the blog explaining it.
I do encrypt my device memory so stuff in transit is safe, and if a potentially malicious software gets in, it doesn't get anything important from an email, but that's that (specially since there isn't malware for BB).