For those using ZCS 5.0.16 and prior, come 15 April 2010 anti-virus definitions will no longer update AND your ClamAV instance will stop working entirely.

Source: End of Life Announcement: ClamAV 0.94.x
Quote Originally Posted by ClamAV Team
Starting from 15 April 2010 our CVD will contain a special signature which disables all clamd installations older than 0.95 – that is to say older than 1 year. We plan to start releasing signatures which exceed the 980 bytes limit on May 2010.
We include ClamAV 0.95 as of ZCS 5.0.18+, there are of course other security reasons to upgrade such as the third-party CVE-2009-3555, in addition to other bugs and multiple RFE's. http://www.zimbra.com/forums/announc...3-shipped.html

If your intent on keeping your old version, you may have trouble with your Amavisd-ClamAV.

Preventative methods:
-Upgrade ZCS.
-Update just the ClamAV component.
-Set zimbraVirusDefinitionsUpdateFrequency to 0 well in advance of that day to avoid receiving the remote disable code.

If reading this already past the 15th date options include:
-Turn off ClamAV from your admin console > server > services 'as/av' tab > uncheck av. Via CLI it's zmprov ms `zmhostname` -zimbraServiceEnabled antivirus. (The minus sign is important, or you'll have nothing but av running.) Then zmamavisdctl reload or zmcontrol stop/start. (This may leave you more vulnerable of course, so schedule a maintenance window as soon as you can.)
-Update just the ClamAV component.
-Upgrade ZCS.

Helpful links:
ClamAV - Updating Version - Zimbra :: Wiki
ClamAV - Updating from versions lower than 0.90.0 - Zimbra :: Wiki
ClamAV - Updating clamd for releases earlier than ZCS 5.0.16 - Zimbra :: Wiki < Freshly written