I feel the need to point out on the subject of Atonement healing: this is, by definition, the first truly hybrid spec. It may not be possible to count an Atonement healer as a full healer, but to dismiss it as such is a failure in your argument. Atonement healers are true hybrids in the sense that they are trading some healing throughput for a fair chunk of dps output.
Do not try to consider Atonement priests in a traditional sense (as healers), but expand your consideration to the idea that they are truly doing two jobs at once: contributing a significant amount of DPS in addition to their melee/tank healing capabilities. Evangelism buffs Holy Fire and Penance damage to a point that it's absurd to not consider using them as DPS talents. I strongly believe Atonement priests will not play the traditional role of Healer, but do seem extremely capable of being the first DPS/Support hybrid class for groups who can learn to use them as such.
For every encounter you've ever done where 5 healers is just not quite enough, but 6 is clearly too many, this is the role Atonement priests may fill very cleanly, and Cataclysm's slower raid atmosphere is the perfect place for such a specialization.
Edit:
My two cents, with a talent distribution heavily focused away from Discipline's Mastery, with Grace in combination with Archangel for heavy single-target healing.