I think the cert is just used by Apache for the SSL handshake... it's not passed along as a header or anything (that would be a big header).
What I've done is to have Apache (in the SSL conf) add certain headers to the request if the cert is verified:
RequestHeader set X_FORWARDED_PROTO "https"
RequestHeader set X_SSL_VERIFIED "true"
and then check for these headers in the rails code.
Fred Kaufman wrote: