Now you can have Protection, Blast Protection, Fire Protection and Projectile Protection on the same piece of armor.
(And Feather Falling if it's boots of course.)
Note that the total protection is capped so that, if I'm understanding it correctly, there is no point in having more levels of, say, Projectile Protection than one half of (20 minus the total number of levels of Protection).
So, if you have 4 pieces of Protection IV armor then one piece with Blast Protection II (or 2 pieces with BP I) will max out your protection against Creepers. And the same goes for Fire Protection and Projectile Protection.
Note that it was less than 2 hours since I posted asking if this was how multiple protection types worked together and now I'm already feeling confident enough in my understanding to tell somebody else that this IS how it works.
I wonder what that says about my position on the Dunning-Kruger scale?