Apple will introduce new versions of iOS and OS X at its annual developer's conference.
Mac OS X 10.5.6 Release Imminent... Friday?
Beyond this, we have heard that Apple has started distributing this particular seed internally outside of the usual developer circles. This would indicate that this is the final version of Mac OS X 10.5.6 before public release. We had also heard unconfirmed whispers that the new version would be made available to the public on Friday, and this new information certainly makes that plausible.
Mac OS X 10.5.6 should be a free update available to all Mac OS X 10.5 customers upon its release.