On the PTR currently, an item's level will determine its base DPS and armor stats, and the monster's level will determine the level of the affixes that roll on the item. There is an exception to this, though:
If an item's level is higher than the monster's level, the affixes will roll at the item's level instead. We felt this was an important exception to make, because players farming Act I of Inferno without any Monster Power may still get ilevel 62 and 63 items from sub-63 monsters, and we want to make sure that those items are still good.