Looking through the forum, I found several OpenVPN threads which may be of interest to you:
http://www.daemonforums.org/showthread.php?t=5653 -- regarding the use of layer2 vs. layer3 tun(4) devices, ARP packets being ignored, and a script to circumvent the problem.
http://www.daemonforums.org/showthread.php?t=3205 -- it includes an older, but working client configuration from forum member There0 which may provide some insight.
http://www.daemonforums.org/showthread.php?t=5625 -- a long thread discussing routing, PF, tun devices, and more. From February of this year.