I have created an owncloud server using RPI, Now I was wondering how to implement the face recognition as a login for the pi. I know I have to use the OpenCV.
A good starting point would be pam-face-authentication. From there site:
PAM Face Authentication is a pluggable authentication module designed for facial authentication. You can use this module for any service that requires authentication, internally handled by the PAM library.
To get started you would need to compile from source as it is not available as a .deb in apt.
Try developing a small opencv program that detects certain faces or shapes and then use it as a login screen (see SLiM) at system startup (could use it as a wrapper for one of those login managers).
I am not sure if there is a project already that deals with this on linux systems, but developing one one seems useful on any linux distro for those interested.