Delta has been using married segment availability for years. However, they did not have any mechanism in place until 01/17/2006 that did enforce it.

Let me give you an example:

Pulling up availability from ATL to LAX via SLC shows (1):
ATL-SLC F9A9Y9B3M0H0Q0K0L0U0T0
SLC-LAX F9A9Y9B3M0H0Q0K0L0U0T0

If you pull up ATL to SLC only (2):
ATL-SLC F9A9Y9B9M9H9Q3K0L0U0T0

If you pull up SLC to LAX only (3):
SLC-LAX F9A9Y9B9M9H9Q9K3L0U0T0

In the past, you could get the Q class seat by booking one segment at a time (steps 2 + 3). Now, it is matched against the married segment availability (1) - and if Q class is zeroed out in the married segment availability display, booking one segment at a time in the lower class will result in an error message - thanks to "Journey Control".