I'm posting this just in case this might help someone else experiencing the same problem.
Although I've received emails before saying "freshclam: Update failed" it normally resolves itself (generally it's caused by a glitch in the network etc.) but beginning yesterday I started getting this email hourly. They all looked similar to this:
2007-04-10 18:48:01.511274500 ClamAV update process started at Tue Apr 10 18:48:01 2007
2007-04-10 18:48:01.787255500 Ignoring mirror 67.15.61.160 (due to previous errors)
2007-04-10 18:48:01.787323500 Ignoring mirror 205.139.192.213 (due to previous errors)
2007-04-10 18:48:01.787365500 Ignoring mirror 209.139.239.158 (due to previous errors)
2007-04-10 18:48:01.787408500 Ignoring mirror 24.215.0.24 (due to previous errors)
2007-04-10 18:48:01.787471500 ERROR: getpatch: Can't download main-43.cdiff from database.clamav.net
[lines above repeated multiple times for each retry]
2007-04-10 18:48:01.798546500 WARNING: Incremental update failed, trying to download main.cvd
2007-04-10 18:48:01.800676500 Ignoring mirror 67.15.61.160 (due to previous errors)
2007-04-10 18:48:01.800744500 Ignoring mirror 205.139.192.213 (due to previous errors)
2007-04-10 18:48:01.800787500 Ignoring mirror 209.139.239.158 (due to previous errors)
2007-04-10 18:48:01.800830500 Ignoring mirror 24.215.0.24 (due to previous errors)
2007-04-10 18:48:01.800892500 ERROR: Can't download main.cvd from database.clamav.net
2007-04-10 18:48:01.833671500 Giving up on database.clamav.net...
2007-04-10 18:48:01.833735500 Update failed. Your network may be down or none of the mirrors listed in freshclam.conf is working. Check
http://www.clamav.net/support/mirror-problem for possible reasons.
After doing a bit of poking around it seems freshclam had marked all the mirrors as bad. (Notice how it says "Ignoring mirror ...."). Running "freshclam --list-mirrors" as root confirmed that all the mirrors it knew about were marked "Ignore: yes". I deleted /var/clamav/mirrors.dat (as root) and manually re-ran freshclam and everything worked.
Hope this helps someone else.
Corey