Our current support policy is to support and bug fix only the latest stable release of CiviCRM. This is primarily due to the amount of time and effort it takes to support multiple releases at the same time. As our user, integrator and developer base increases we expect this policy to change.
Both Drupal 6 and Joomla 1.5 are fairly active releases with support for those releases expected to last for another 12 months or more (based on when the next version is released). However the next version of CiviCRM, v4.1, will only support Drupal 7.x and Joomla 1.6.x based on our current support policy.
We do understand that a large number of our users will still be on the older versions of Drupal and Joomla. If you are not planning to upgrade to the latest version of Drupal/Joomla, please contribute to the Make It Happen that will allow us to extend support for Drupal 6 / Joomla 1.5 for another release. This will ensure that these releases will be supported at least till Mar, 2012. Based on our current schedule, Drupal 6 / Joomla 1.5 will not be supported from Sep, 2011.