A new major version of Drupal is about to be released so at some point we should upgrade to it. Our Drupal site (along with most others) rely on addition modelules. Releases of Drupal are a bit messy because of the high degree of dissagregation of the development. The core is publicly released before most modules to give a stable platform for module writers. Core 6 is now released (and patched) but modules vital for most sites, such as views and CCK (see below), are still being worked on.
We should release our Drupal-based site using version 5.x and then upgrade to version 6.x when all the modules we need are ready.
Contributed modules status - version 6.x this seems a bit out of date. The projects modules are probably the only thing we need and a development version is being used on drupal.org but the upgrade path from that is likely to be difficult.
JMM 7/7/09