Some changes to Ulduar have been hotfixed into 3.1.2 or are in the process of being hotfixed. Daelo provided the full list on the official forums, and they include changes to trash, bosses and hard modes. For example, here's the hard mode update:

"The hit point increasing hard mode ability for XT-002 Deconstructor has been reduced to 50% in the 10 player Normal setting, and 60% in the 25 player Heroic setting. The corresponding damage increase for XT-002 has also been reduced to 15%. Life Sparks also have half of their previous hit point values."

You can read all of the changes below and discuss them in this thread in our WoW general forum.

The following are a list of changes that have been hotfixed to 3.1.2 or are in the process of being hotfixed. All changes are to both 10 and 25 player versions of the instance, unless specifically noted.

Trash
- Storm Tempered Keeper’s Supercharged ability will no longer increase the damage of Storm Wave.
- Some Ignis and Freya trash has been removed. If your instance was already open when the hotfix was applied, the trash will still be there.
- Ice Turret’s Ice Nova spell now does considerably less base damage.
- Rune Etched Sentry’s Flaming Rune does does considerably less damage over time.
- The Molten Colossus will no longer cast Unquenchable Flames.
- Chamber Overseer’s Staggering Roar does considerably less damage.

Bosses
- Kologarn’s Crunch Armor effect of his Overhead Smash now has a greatly decreased duration in the 10 player Normal difficulty setting. This change negates the explicit need to tank swap Kologarn.
- Thorim’s Chain Lightning no longer affects more than 8 targets.
- General Vezax now does 10% less melee damage in the 10 player Normal difficulty setting.

Hard Modes
- The hit point increasing hard mode ability for XT-002 Deconstructor has been reduced to 50% in the 10 player Normal setting, and 60% in the 25 player Heroic setting. The corresponding damage increase for XT-002 has also been reduced to 15%. Life Sparks also have half of their previous hit point values.

In an unrelated tuning note, we also found a case where Algalon would not reset properly in the 25 player Heroic setting only. This has been hotfixed.