Blizzard's Patch notes sometimes provide data on the drop rate for an item level range in a specific Act/Difficulty.
For example, the 1.0.3 notes provided this information :
High-end items (items level 61-63) will now drop in all Acts of Inferno and Acts III and IV of Hell difficulty The new approximate drop rates are as follows: Hell - Act III and Act IV iLvl 61: 9% iLvl 62: 1.9% iLvl 63: 0%
I'd like to know exactly this information, but for items from 1 to 63, and all Acts/Difficulties. I find myself trying to decide between playing in an Act that is really hard for my character, or going back to something easier to grind for gear. It would help to know if the gear from previous acts is likely to help me, or if I would just be getting items too low to use.
Best Answer
The Official Brady Games Diablo III guide says this about the loot system.
It also goes on to say;
This is why the patch notes you quoted say "The new approximate drop rates are as follows" since drops from mobs are not based on act but monster level.
When I have some time I'll compile a list of the level ranges for each act/difficulty since the guide does have a complete list of the bestiary complete with monster levels.
To be honest I think you're over thinking it a bit. If you're level 60 then you should be farming the highest act you can reasonably do no lower than act 3 Hell mode and if you're lower than level 60 then farm the highest act you can farm efficiently. Farming one act back from whatever act is giving you trouble will give you a chance at gear you can use.
edit
There are some higher (and lower in act 2) level mobs in both acts 1 & 2 but not many. Act 1 has one ilvl 63 (spiderlings) for instance.