I came to think of this idea when watching an episode of Pokemon. Anyway... so for example if I find an item(of quality) it will get listed. How we can proceed from this can be many different ways. I will try list as many i can think of and add some pros and cons.
All content within the "Gameguide > Items" section here at battle.net copied to the ingame database.
Much easier and more comfortable to access this when playing instead of open a new tab in respective web browser and come to battle.net all the time.
May help solve the issue that people cannot tab the game, now they got one less problem, they wont be needing to shut down the game just to access the items at battle.net.
Having you find two similar items and not be able to choose yourself which one to play with and also which is best for your class, the database can make that choice easier for you. Save the better and sell/scrap the other one.
None that I can think of.
For each item there could be a number of possible stats combination, if this number is fixed... finding an item that maybe is the second best item will list number two. We may know the items placement but items that haven't been looted yet wont be shown or listed.
Many causal player without the pure knowledge or time to remember all the best items and their stats will have a lot easier to find the best suited items for their respective class.
Listing items like this may cause trouble with items becoming "useless" or worthless as we eventually know their placement in the item database.
Possible to access from without the game via your battle.net account to access your personal database, this should be hidden from other players to avoid taking shortcuts.
When not able to play the game this can be used to inventory your found items.
If not made permanent private this could be used to find players profiles to take shortcuts without making their own progress ingame. I.E people will depend more on others and the main concept of private content is destroyed.
Showing items in the database you have seen at the auction house wont be possible, you have to own the item before it shows in the item database.
To avoid abuse this would be necessary, create a bigger and far interesting risk or chance of obtaining greater items.
It contribute to selling items more easier to obtain a full database, i.e it would in theory increase overall selling rates at AH, making it easier for people to get items sold.
As stated above when items sell more easier you save inventory, you save time and as you only for now got maximum of 10 selling slots, you don't have to worry as much as if this idea in 2.2 wasn't there.
May contribute with creating a more stable market as if items become more coveted, this to obtain a complete database, it gives the player a hard choice!
The choice is ether to sell or scrap the item for salvaging purposes. You need both gold and material from salvaged items to create new ones. This in hand wont give the problem of over-flooded salvaged materials roaming the auction house as it currently is in the beta. I.E price will go up as the demand of specific items rise. The perfect balance!
For obvious reason it gets harder to obtain item information, this may frighten people with limited resources(gold or real money) to take risks(as suggested in idea 2.2). But to win you must play the game.
Why not also have an achievement that follows your progress of the database, for example the ultimate achievement would not be from a boss fight or that single legendary item everyone wants. But it would be the complete database of all the items you ever possessed. That my friends, this is a real hardcore achievement not to take lightly, one you would love to have and brag about.
May take a very very long time
Please take all ideas with a tip of salt, let me know what you think, if you believe this is a good idea or maybe just something you wont be needing. Please discuss around this topic!
Ultimate respond would be to have Blizzard Diablo3 developers giving their respective thoughts regarding this topic.