My reasoning isn't to nerf Holy Paladins as healers. I love all healers equally. Rather, my idea here is to incorporate a class design that's been picking up steam since the end of Burning Crusade. Back before patch 3.0 dropped, two Paladin specs wore spell plate, Holy and Protection. Since that was two specs out of a possible six plate wearing talent specializations (2/6 = 1/3 = roughly 33 percent, I'm not doing a repeating) that meant that there was a fairly sizable minority that wanted the stuff.
Flash forward to Wrath of the Lich King. There are now three plate wearing classes. Two of them (six total talent specs) have absolutely no use for spell plate. The one class that does use spell plate? Well, the Protection tree was revamped, so tanking Paladins don't wear it. Retribution has been giving spell plate the stink eye forever. So we now have 1 out of 9 possible specs wearing the stuff, and yet it drops everywhere. There's no reason spell plate needs to exist, much less be a protected species by Blizzard when they went out of their way to do away with tanking leather, for example.
Of course, what will Holy Paladins do without spell plate? Much as Protection Paladins needed a means to keep their mana up and their threat intact when they switched away from the stuff, Holy will need talents and abilities that can convert the stats on physical plate (we'll assume DPS plate, as it seems weird that Holy would use tanking plate) to healing power.
Frankly, the Holy Paladin is an anomaly in the way Paladins play. Both Ret and Prot paladins wade into the thick of the fray while Holy is forced to stand around in the back and heal. Combine this with Holy's unique healing niche of being a primary tank/single target healer (to be fair, Beacon of Light does provide some group healing utility, and I'm sure dedicated Paladin healers could detail far more) and you end up with a class that tends to stand in one place spamming heals on one target a lot of the time. There are, however, ways that you could convert the Holy Paladin into a healer who actually ran into melee in order to provide his heals.
The first thing that comes to mind is changing Divine Intellect so that it converts a stat like Strength or Stamina into Intellect rather than simply adding more int by percentage. Then change Holy Guidance so that it grants a percentage of Attack Power as Spell Power. Then what? Well, we'd then need to expand the power of Seals and Judgments so that a holy paladin in full DPS gear could run into melee, unload his Judgement on a target and it would convert the damage he or she would normally deal into healing for his or her group or raid. There's lots of ways this could be accomplished - a talent like Beacon of Light that you placed on a target that directly converted the damage the Paladin would deal into AoE healing, perhaps, or a beefed up Judgement of Light that stacked with the Paladin's attacks to provide really significant group healing but which required the Paladin to keep attacking to keep the stack up. There could be a specific Holy Paladin Aura that provided a small but noticeable group heal or even a Heal over Time effect on a designated target (think Blessing of Renewal) where the damage the Paladin deals creates healing on those with that specific blessing. You'd have to watch how the numbers work: either the Blessing provides a fraction of the Paladin's DPS or the Paladin's damage is instead converted to healing, so that while he or she has to stay in melee range she's still providing healing and not actual damage to the group or raid.
This is just one path, of course, and I have neither the skill nor the resources of the entire Blizzard dev team who can come at this problem from all sorts of angles I'm aware I'm missing. Still, however they decide to solve the issue of spell power plate on a tanking class in PvP and the unexpected healing power it gives, I'd really like to lobby for the complete and total annihilation of all spell power plate. Destroy! Destroy!