EDIT, 2/18/13: These calculations were made 2 weeks after MoP's release and are largely irrelevant to current gear levels and content.
Glyph of penance is being used in value calculations. No buffs (no inner fire or inner will) used for calculation. Divine aegis, spirit shell, train of thought, and talents not accounted for. Assumed that inner focus is only used for greater heal or prayer of healing.
Smite remains a bad choice to use for healing, even as gear increases. Use it only if you have to help out with DPS for burn phases and you can spare the mana.
While holy fire is not very efficient without evangelism, it becomes a good atonement heal once evangelism fully stacks. You should use holy fire often enough to maintain 5 stacks of evangelism.
Power word: shield becomes significantly more effective the more spirit you have. If you can proc rapture by breaking the shield, it's our second-most efficient heal and, with inner will and/or enough spirit, our most efficient heal.
Renew is not as bad a spell as it used to be, though it's really only efficient to use if borrowed time or some haste buff putting you into a 5th tick is available. So, after using power word: shield, use renew before using up the borrowed time buff on another spell.
Based on this chart, your basic single-target healing priority would be:
Power word: shield if the target will take enough damage to break it and proc rapture.
Renew while borrowed time is up.
Penance if the target needs it.
Holy fire to maintain evangelism.
Heal otherwise/maintain stacks of grace.
During high damage phases, you would swap out heal for greater heal/inner focus. Alternatively, you could do spirit shell/heal, and if damage is especially high, spirit shell/greater heal. Once spirit shell has worn off, if you still need extra healing, clear your stacks of evangelism for archangel.
For AoE healing, keep prayer of mending up, then use prayer of healing for 5-man groups. For high AoE damage, spirit shell/prayer of healing. If at least 7 targets need healing, use your level 90 talent. Prayer of healing is more efficient for all 5-man content than any of the level 90 talents.
Interesting note about gear scaling
If you compare the no gear chart to the wearing gear chart, you'll notice that, although the values change (the spells become more efficient), the efficiency priority barely changes. Only holy fire (5 evangelism) and power word: shield shift around to any significant degree. It looks like, at least for now, your basic healing rotation will not change regardless of gear.
Zevia - appreciate the work you did putting this together. However, I believe you really aren't calculating the full benefit of smite or holy fire. I'll throw out some simple and low numbers to illustrate my point.
If a mob has 160k health - and your holy fire or smite hits for 40k... then that 40k is 40k heals AND reducing the mobs health by 25%. The effect in a real raid scenario is that for every point it heals it's also being applied to the total damage of the boss. This is obvious I'm sure - but what that really means is that you are contributing to reducing the duration of the fight which means the fight requires less healing. You could almost think of it as the shield in that it's reducing the amount of damage taken with a shorter fight.
As a discipline priest, without any effort at all, I'm easily contributing between 5%-8% (~1 to 1.5 million) of the raid damage done to a boss with just smite and holy fire. Keep in mind its not how I heal the fight - but when times permit and during light damage on the tanks - that's what I'm healing with because it by far out performs any other spell I have.
This is because smite is as fast as flash heal to cast while costing just a little more mana than heal. A smite with a glyph'ed instant cast holy fire can often heal for as much as a greater heal for just over 1/2 the mana (holy fire costs the same mana as heal).
There are other benefits - such as a 70 yard ranged smart heal (30 yards for smite/holy fire + 40 yards for healing the lowest damaged player) that numbers just can't account for. For example, this becomes invaluable when you need to heal that pheromone kiter and the tanks on the other side of the boss. Another unseen benefit are those boss mechanics which GREATLY increases dps - at which times I'm hitting for 200-300k a smite or holy fire while healing the same. No heal that you list can outperform that.
In my opinion - all discipline priests should always keep a spot for smite and holy fire on their cast bars.
I made that post back when I had recently hit 90 and was trying to figure out a proper rotation according to what would be most efficient. At current spirit levels for 5.1 (I run about 9050-9100), mana is such a non-issue for disc priests that it hardly matters what's efficient (as long as you aren't spamming flash heal or binding heal).
After clearing all normal mode bosses and a handful of heroic bosses, I'd say the rotation is to basically just penance on cooldown, holy fire on cooldown, smite for filler, and then use your various cooldowns as necessary. Even when 2-healing fights with the resto druid in my guild, the vast majority of my time is spent on atonement healing until there are large bursts of damage, at which point spirit shell, prayer of healing, or greater heal might start making appearances.
I don't think there has ever been a time that I felt I needed to stop casting or conserve mana on this rotation. The list, while useful when I first hit 90, is already extremely outdated at 4 months old.