Currently, References are part of Statements. I propose to make them top-level members of the Item, and reference them from statements. This has several implications:

• references would be shown much like Wikipedia does it with the cite extension: a small [2] at the Statement, referrfing to a source given at the bottom of the page.
• the UI for editing a statement would be come much simpler, die display could be made a lot more terse (no extra line for "no references", etc).
• references can be re-used in multiple statements on the same item (global re-use is a separate issue). This is convenient for cases where several statements are cited from the same source, which is common enough for e.g. census data.
• references need to have a stable guid instead of a hash, so pointers to references don't break when they are modified.
• reference guids should be qualified with the item's id, just like statement guids, so they can be used as locators.