Update (09/04/2009): Archon 2.23 has been released which fixes the PHP 5.3 compatibility issue. It is recommended that any current users upgrade as soon as possible to prevent problems when your webhost upgrades to PHP 5.3.
PHP 5.3 introduces a number of significant changes, some of which cause Archon 2.22 to stop working. We have written a fix to make Archon compatible with PHP 5.3, and will be releasing 2.23 in the very near future to address this issue (note that the only change in 2.23 will be fixing this issue, so anyone running 2.22 can upgrade when it is released without worry). In the meantime, you will need to make sure that your web host is running PHP 5.2.10 or earlier. Also, if you are currently running Archon and unexpectedly start receiving error messages, check to make sure your host didn't upgrade to PHP 5.3 without your knowledge as a first step in diagnosing the issue.
If you are not responsible for managing your web server, or you are not running PHP through IIS as an ISAPI filter, you can stop reading now. I wanted to mention that one other important change PHP 5.3 introduced is that a compiled ISAPI module is no longer being distributed in the standard PHP package. This means that for 5.3 and beyond, you'll have to modify your IIS configuration to run PHP in FastCGI mode. See here for more information.