Blizzard has partnered with iTunes to offer the “Diablo III Overture” by Russell Brower & the Eminence Symphony Orchestra, as a free download. You must have itunes installed and an account with the Apple Store to download it. The tune is 3:46 in length, and parts of it should sound familiar from the WWI cinematic trailer. Click the link, and when itunes opens go to the “Apple Store” tab and the download should display there, along with the file information.
Update: Readers report that this song can only be downloaded with a US itunes account. We’ll add a link for international Diablo fans when one becomes available.