Just finished (codewise) and in the game:
Priests - A new specialist who generates extra piety when working in Temples / Monasteries etc. Collect great people points toward:
Great Prophet - Yep, they're back!
Abilities:
Start Golden Age - Golden ages also increase the range and/or strength (balance determining), of your piety emission, making it easier to convert other civs.
Build Shrine - The GP's great improvement. It can be built in neutral terrain as well as in your borders. Has a range of 5 hexes and adds 5 piety to any city within range, yours or other players cities. Instead of missionaries (at least until the SDK), your new way of attacking far away civs with your religion is to send a great prophet out to build a shrine near to their borders, these will then start transmitting piety to any cities in range over the border every turn, though taking an armed guard is a must, not only to protect the GP, but then you'll need to station guards at the shrine indefinitely to make sure it's not torn down by another Civ.
Other Civs can counter this without declaring war by getting control of the tile the shrine is in, giving them ability to eject the unit and pillage / replace the improvement.
They can also be used in your own civ borders to bolster your own cities piety production (the only way it will count toward your doctrine unlocks)
This also allows for interesting combos, since your own cities get bolstered by the shrine, they will become more effective at transmitting their piety to other player's cities. If you get a shrine in range of your own cities and your opponents, then they will not only get the piety from the shrine, but also increased piety from the cities that shrine strengthens. This makes shrine placement very important and strategic.
Splinter Religion - If you have a state religion that you're not in control of (was founded by someone else) you can use the GP to break off your own religion using theirs as a basis. Your new religion will still retain the same holy city, and all the doctrine unlocked to that point, but then you can develop it on your own. You can also edit the name by adding a prefix or suffix to the original root name to make your offshoot religion.
Art coming soonish.
Also: I edited the main post with more details on the religion spread system.