Inferno Mode Item Tiers
Unlike the previous acts, where tiers of items were based on monster level within that difficulty level, things are a bit different when it comes to Inferno Mode items. Here is how it works:
Act 1: This act can drop items up to tier 16. Monster levels here are level 61.
Act 2: Drops up to tier 17. Monster levels here are 62.
Act 3 and 4: Drops up to tier 18. Monster levels here are 63.
This means if you want Tier 18 items (the best base stats in the game), you have to get them from Acts 3 and 4 of Inferno Mode. They will not drop in Act 1 or 2 or in Hell Mode despite an old Blizzard post that said they would. The latest Blizzard info says this is how Inferno drops break down.