Doomscream pointed me to found on the European Battle.net server on MMO Champion, where they credit a fan for discovering the image you see to the right. This image can be found on the European Battle.net server, along with the other game box artworks that show up in your Battle.net account.
To make that clear; if you are accepted into the Diablo III Beta test, you’ll see this icon on your Battle.net account, just as you now see box icons for WoW, Starcraft II, D2, etc. Whichever games are attached to your account.
This image doesn’t mean anything special; obviously it’s not the final game box art, and we knew Blizzard would have to create some sort of icon/image for this purpose, and we knew it would be online fairly soon… but it’s still damn shiny to look at. Also, soon!
Incidentally, just this afternoon I recorded a Diablo Podcast with Exile and Neinball, during which we discussed all things beta. How to get into the test, how many people will test, how long it’ll run, when it will begin and end, what Blizzard is looking to try out, what the D2 and other past Blizzard tests were like, and much more. It should be online Monday, so check back for that.
In the meantime, you may use the comments to SQUEEEEEEEEEEEEE over this box art, if you so desire.
Update: In comments, RisingRed and CherubDown pointed out small and large versions of what looks like a D3 desktop icon, also found on Battle.net. You see the image to the right.