I could not believe they would use single DES (I would have expected AES), so I googled around. Apparently (http://stackoverflow.com/questions/6312871/what-encryption-a...; it is (or at some time was) 3DES; the PDF linked from there states:

   "All the password data in the keychain is protected using the
    Triple Digital Encryption Standard (3DES)."

http://en.wikipedia.org/wiki/Triple_DES#Security states:

   "NIST considers keying option 1 to be appropriate through 2030."

I am still surprised that it is not AES, but 3DES seems good enough. Also, I am not sure that PDF still describes the current situation.