With the new patch, the cooldown for perma-ghouls stays at 100% until the ghoul dies.... OR if it disappears. This is an important fact. If you simply mount while your ghoul is around, your ghoul will disappear and the cooldown will be enacted on ghoul.
The end effect is that you can now go into a fight with a fresh ghoul to summon instantly if yours dies (or if you need to use death pact to heal yourself). Normally, if your ghoul would die in a fight, you'd have to wait for the cooldown. Thats a lot of dps missed out on. Anyway, here are a couple screenshots:
You can see that I have a ghoul up, while the ghoul button being off cooldown.
You can see the old ghoul fading out and the new ghoul coming up at the same time.
Also further proof, notice that the names of my ghouls change.
This helps after wipes in Ulduar (while running to the teleporter, summon your ghoul and then mount) and for all other Unholy DK's that have access to outside where they can mount.