Anomie, thanks for your reply. I retook this problem. See, when i try to generate the pkcs from the private key, it works. However, when i try with the private key, it complains. See:
$ openssl rsa -outform DER -in id_rsa -out id_rsa.priv.pkcs
writing RSA key
$ openssl rsa -pubin -outform DER -in id_rsa.pub -out id_rsa.pub.pkcs
unable to load Public Key
11146:error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c:647:Expecting: PUBLIC KEY
Both files (id_rsa and id_rsa.pub) where generated in a single ssh-keygen execution. Any idea what I'm missing?