During Beta:

Frost Presence is +60% armor, -5% magic damage taken.

WoW 3.0.8:

Frost Presence is +80% armor, -15% magic damage taken

WoW 3.1.0:

Frost Presence is +80% armor, -5% overall damage taken

WoW 3.1.3:

Frost Presence is +60% armor, -5% overall damage taken

Conclusion:

The change from -5% magic damage to overall damage was sufficient from beta to now. The major reason for the original changes came from a few other things:

- Launch DK needed a lot more defense to be uncrittable. In the intervening time, DK gained Rune of the Stoneskin Gargoyle and Sigil of the Unfaltering Knight, and no longer need massive defense socketing. This allows them to skew itemization towards massive stamina pools, which get multiplied significantly.

- Launch DK needed either the 20% bonus armor or the 5% damage mitigation. In addition, Blade Barrier changed from 10% parry to 5% all damage reduced. This combines to effectively form 'defensive stance' for DKs (like the other tanks have), but they also had the extra bonus armor that provides more armor than any shield.

--Rawr