Prior to Patch 2.1.0, Illumination granted a 100% mana gain after a related crit. While 2.1 was being tested on the PTR, the "new" Illumination talent was initially 50% before being adjusted to 60% for release. This resulted in many Holy-specced Paladins who had been focusing on spell critical strike gear to change their focus to equipment that provided more mana per 5 seconds.
Patch 3.2.0 (04-Aug-2009): This talent now returns 30% of the mana cost of the spell instead of 60%.
Patch 2.1.0 (22-May-2007): This talent now only gives 50% of the mana cost of the critical heal. It also now returns the correct amount of mana when used with ranks 4 and 5 of Holy Shock.