The other night I got to DPS in Icecrown Citadel. When we got to Valithria Dreamwalker, I decided to try healing her specced as Retribution but in Holy gear.
The key talent that makes this idea possible is Sheath of Light. You get a really powerful HoT ticking away on Valithria, one that lasts for 12s while you are in the dream world. The other advantage you get is that Sanctified Wrath might let you get off an extra Avenging Wrath.
The raw total was very close to the healing I do as Holy, if not slightly higher. Holy Light did 54% of my healing, while Sheath of Light did 46%.
Of course, my Ret build is not designed for this. It is a regular Retribution build with DPS glyphs. You could do something like 23/5/43 and run Glyph of Seal of Light to completely maximize Dreamwalker healing. But it's really not necessary. It's just a fun thought experiment.
The other advantage that the Holy build has is that you can cast Beacon on Valithria and add extra healing to the raid. I've found that doing this is valuable at the very end of the fight, keeping the raid from being overrun.