I can confirm that changing the interval to 180 fixes things. It takes approx 100 seconds on my PE2650 (2xXeon 2.4GHz) server for clamd to start up. If it doesn't start up in the interval specified in zmmtaconfig, it will get re-started again and again.
I didn't upgrade zimbra's clamav to a newer version or anything. This is a RHEL4 system, and I do have a regular clamav rpm installed, and I could use that instead. Rather, I just opted for redhat's package to start up freshclam and not clamd, as there is a local copy of havp/squid combo to virus-filter the web traffic (this is our mail/squid server).
I presume it wouldn't be such a big deal to have zimbra just use the distribution-provided clamd, but there's just no need -- havp uses libclam only and there's no need for redhat's clamd running, only zimbra's and it works. Slow startup, but works nevertheless.