If you decide to buy a digital version of World of Warcraft: Mists of Pandaria, you have an opportunity to get some exclusive extras, too. Battle.net is now listing a Digital Deluxe Edition of the expansion pack.
The deluxe edition is mentioned in passing in the Battle.net account management page (via MMO Champion). Downloadable versions of Blizzard's games are generally exclusive to their own store. The same would probably apply to this special edition.
There's no mention of the price or contents of this bundle. The boxed collector's edition of the previous WoW expansion, Cataclysm, came with an art book, behind-the-scenes video, soundtrack and mouse pad. Everything except the mouse pad seems plausible for Pandaria's digital deluxe offering. They're standard accessories in any limited edition, after all.
Expect a token for some sort of in-game content, too. For example, the Cataclysm CE came with a free in-game pet. Blizzard has shied away from selling in-game bonuses in the past so the only game content they'll offer will likely be limited to cosmetic items like mounts or pets.
Blizzard's games unlock at 3 AM PST on their respective launch days. Digital copies are especially convenient, then, because they save hardcore players a trip to the mall in the middle of the night. It's a no-brainier to offer die-hards a way to both play at launch time and get some interesting extras in the process.
Pandaria is expected to launch later this year. It's been in beta since March.