This is not a tanking glyph; tanks will have enough defense that they will have at least 40% damage reduction from Icebound Fortitude already. This glyph is only useful for PvP.
Patch 4.0.6 (2011-02-08): Now breaks stuns when used, in addition to providing immunity to them.
Patch 4.0.3a (2010-11-23): Now provides 20% damage reduction, down from 30% (now 50%, down from 60% when talented), and has a 3-minute cooldown, up from 2 minutes.
Template:Patched
Patch 3.2.0 (2009-08-04): Cooldown increased to 2 minutes.
Patch 3.0.8 (2009-01-20): Now reduces damage by 20% instead of 50%. The amount of damage reduced increases with bonus Defense (to about 35% for 540 Defense, but can go higher).