Apple has never had a public support policy, but that's the way it's always worked.
Current version (call it 10.x) gets point upgrades (eg. 10.x.1, 10.x.2...) which may add features or fix non-security bugs, as well as security updates. That's Lion now.
Previous version (10.x-1) gets security updates only, no other fixes. That's Snow Leopard now.
Older versions (10.x-2 and older) get nothing. That's Leopard now.