Drupal 8 is currently slated for sometime Winter 2013 (which, to me, likely means early Q1 2014). Once D8 is released, Drupal 6 will no longer be supported.
That means no bug fixes. No security updates. Nada. Your site will be running on unsupported code. You don't want that.
UPDATE: IT'S CURRENTLY UP IN THE AIR IF D6 WILL HAVE LTS OR NOT. See this issue.