I'm increasingly confident in Drupal 4.1.0's release readiness. As such, chances are we'll release Drupal 4.1.0 within two weeks. I've seen quite a few production sites that use the Drupal 4.1.0 release candidate, yet it is desirable to have it tested in a diverse set of setups. If you have a Drupal 4.0.0 site and you want to help quality assurance testing, consider upgrading now. We tend to be much more responsive to bug reports and upgrade problems _before_ the actual release, so there is little to loose (as long you have a backup, that is).
Please report back after upgrading; good or bad. Thanks.
(Note that the modules and themes in the contributions repository might be dated and that they are not officially supported. To the authors and maintainers of such modules and themes; this would mark the appropriate time to put the champagne aside and to update your work.)