sshd does not appear to read authorized_keys2 during connection attempt from ssh client. This was verified on both a Solaris system running SSH-2.0-OpenSSH_2.9p2 and a Linux system running current version of sshd. Resolution of the problem was accomplished by creating a link from authorized_keys2 to authorized_keys. Current manpage does not mention authorized_keys2, and the ChangeLog seems to indicate authorized_keys2 has been deprecated, yet it appears to still be required for Protocol 2 connections.
authorized_keys2 is required if you are running an old sshd (<2.9.9 IIRC). Newer sshd's do not require authorized_keys2, but still check it for backwards compatability - it is certainly not a requirement.
Mass change of RESOLVED bugs to CLOSED