Patch 6.0.2 (October 14, 2014): Shadowform no longer allows any healing spells to be cast but also no longer has a restriction on casting other Holy spells.
Patch 5.4.0 (10-Sep-2013) Shadowform now increases the Priest's armor by 60%, but no longer reduces all damage taken by 15%.
Patch 5.2.0 (5-Mar-2013): Shadowform now increases the Priest's Shadow damage by 25% (was 20%).
/Patch 4.0.1 (12-Oct-2010): [Cure Disease] can no longer be cast in shadowform.
Patch 3.3.0 (08-Dec-2009): This talent also now causes Devouring Plague and Vampiric Touch to benefit from haste. Both the period length and the duration of these spells will be reduced by haste. In addition, the mana cost has been reduced from 32% to 13% of base mana.
Patch 3.1.0 (14-Apr-2009): Now reduces all damage taken by 15%, not just physical damage. Bonus damage from critical strike chance removed and replaced by the ability of those periodic damage spells to generate critical strikes.
Patch 3.0.8 (2009-01-20): You can now shift into Shadowform while mounted or sitting.
Patch 3.0.3 (04-Nov-2008): Shadowform: Devouring Plague, Shadow Word:Pain, and Vampiric Touch cast in Shadowform deal increased damage percentage equal to the player’s chance to get a spell crit on their target.
/Patch 3.0.2 (14-Oct-2008): Now has an innate 30% threat reduction. Now has a shapeshift UI.
Patch 2.4.0 (25-Mar-2008): Fear Ward is now usable while in Shadow form.
Patch 2.3.0 (13-Nov-2007): Fixed a bug where the global cooldown was triggered when shifting out of Shadowform.; If you are in Shadowform and try to use an ability that my not be used in Shadowform, you will leave Shadowform and use that ability. This can be disabled with /console autoUnshift 0
Patch 2.0.3 (09-Jan-2007): The visual effect of "Shadowform" will no longer persist if disabled while mounted.
Patch 1.7.0 (13-Sep-2005): Switching to Shadowform no longer cancels Holy buffs on a priest.
Patch 1.6.0 (12-Jul-2005): Changed so all spells except Holy can be cast rather than only Shadow and Discipline spells. Tooltip updated. This fixes several bugs that prevented various actions (creating tradeskill items, mining, summoning a mount, using a hearthstone and many more).