During the Burning Crusade expansion, "Flash of Light spam" was a popular style of healing for holy paladins in PvE. During the early phases of Wrath of the Lich King Flash spam fell out of fashion due to the nearly unlimited mana made possible by Divine Plea and Illumination. As the expansion progressed, there were several nerfs to Paladin mana regeneration. With the mana nerfs, the increased availability of high spell power gear, and increased availability of sockets allowing further spell power increases, Flash of Light spamming re-emerged in PvE healing. There is considerable disagreement over whether Flash of Light spamming is sufficient for the tank healing niche of Holy Paladins in raids, but it appears some paladins are using it successfully.
Patch 4.2.0 (2011-06-28): Flash of Light mana cost has been increased to 31% of base mana, up from 27%.
Patch 4.0.6 (8-Feb-2011): Flash of Light mana cost has been increased by approximately 10%.
Patch 3.2.0 (04-Aug-2009):Sacred Shield: When a paladin casts Flash of Light on a target with this buff, they also now place a heal over time effect on the target, healing that target for 100% of the Flash of Light amount over 12 seconds. The Art of War: Now only applies to melee critical hits, but will make your next Flash of Light or Exorcism instant. Exorcism has a cast time of 1.5 seconds but can be used on players again. ...we are changing The Art of War to make not only Flash of Light instant, but also Exorcism. Choose healing or damage. (source)
Patch 3.0.3 (04-Nov-2008):Infusion of Light now affects Flash of Light too, reducing its cast time down to zero and can be used while moving.