We're now working hard to adapt the Active Agenda source code to work under PHP 5. We plan to support PHP 5 in the next release, which will be re-numbered as 0.9 in recognition of this, pretty major, new feature.
In the meantime, if you'd like to check out how well (or not) Active Agenda runs under PHP 5, it is possible to use the database files from the current 0.8.2 release and updating the source code to the latest code available in the SVN code repository. See the following article for some more pointers:
http://www.activeagenda.net/documentation/index.php?title=Getting_the_latest_code