i'm getting this same problem on 3.0.0_M4_6.DEBIAN3.1-cvs and 3.0.0_M3_381.RHEL4-developer builds. install/upgrade goes great except that saslauthd fails to start because the mech url is blank in cyrus-sasl/etc/saslauthd.conf, and cyrus-sasl/etc/saslauthd.conf.in line reads:
zimbra_url: %%zimbraMtaAuthURL%%

because i usually move the port 80 user interface off to another port like 85, i put:
zimbra_url: https://localhost/service/soap/
in saslauthd.conf.in and this fixes the problem for me. hope this helps.