Patch 4.0.6 (8-Feb-2011): 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.
Patch 3.3.3 (2010-03-23): Tooltip corrected to indicate it reduces damage by a minimum of 30%.
Hotfix (2010-02-05): Icebound Fortitude now provides 30% base damage reduction, up from 20% damage reduction. For a geared tank with high defense, this translates to 50% damage reduction, up from 40%. Tooltip to be fixed next patch.
Patch 3.2.0 (04-Aug-2009): 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).