UPDATE 16/02/2015 4.43pm: The Xbox Marketplace listing for Dark Souls 3 - including the pre-order bonus of backwards compatible Dark Souls - has surfaced for the UK. The core game costs £49.99 or you can snag the Deluxe Edition with the Season Pass for £69.74.
ORIGINAL STORY 15/02/2015 11.10pm: Dark Souls has been revealed as a digital pre-order bonus for Dark Souls 3, indicating that From Software's cult classic action-RPG will be receiving backwards compatibility support sometime before its successor's launch on 12th April.
The news was spotted on the Australian Xbox Marketplace by astute Redditers. The listing has since been removed, but it noted that the code for Dark Souls would be delivered within 7-10 days of purchase.
With any luck it will improve some of Dark Souls' technical foibles like its framerate, which really took a hit in its original console release.