Well, got duo upgraded again. This time to RedHat 9. There were some minor difficulties getting services running again, but overall I think things went pretty well.
AFAIK, the only service currently not running is the MSN Transport for Jabber. I'm not even sure if that affected anyone. [Brady] had some difficulties with his website and PHP's default setting for register_globals in 4.2.0+, but we got that ironed out as well. Overall, I think the upgrade was a success. Brady can even use SMTP+TLS now; excellent!