is it possible to kit out a lot of Ubuntu machines with antivirus protection, then have them point to a central update/administration server?
You'll want to investigate ClamAV. I don't know if it's centrally manageable but I'm sure you can at least use a centralized update server.
Note that ClamAV mostly detects Windows viruses. It's primarily intended for use in mail/file servers for Windows clients. There are almost no Linux viruses and the ones that do exist are not a significant enough threat to warrent any attention.
ClamAV is the de facto standard antivirus program for Linux.
For updates, if you're using Ubuntu and Debian, ClamAV updates are distributed just like any other package updates (see here or here), so apt-get will take care of downloading and installing them for you. If you really want to control updates yourself, you can set up your own repository using any of the howtos online (such as this one).
For other aspects of centralized administration and reporting, you'll want to look into general Linux administration tools (like Puppet or Cfengine) and loggers like syslog-ng or rsyslog. All of these can be configured to handle ClamAV as well as everything else on your boxes. But that's becoming more of a Serverfault question.
There are commercial products available - McAfee, for example, offers LinuxShield - but I doubt they offer much over ClamAV.
Yes, as many people have pointed out, ClamAV is the de-facto standard... mainly because its free. However, its never done that well in the anti-virus tests.
With the growing popularity of Linux, there are Linux AV products available for purchase from the established players. McAfee LinuxShield (thanks Josh), Trend Micro's ServerProtect, Sophos Anti-Virus for Linux, and others