It won’t be native; inserting a supported 360 disc will reportedly trigger a download of an Xbox One-compliant version, while digitally purchased last-gen content should work right off the bat. It’s probably safe to assume that any discs used to initiate said downloads will need to remain in the drive during gameplay. Once individual publishers have given a nod of approval in terms of including their titles in the initiative, Microsoft will, in their words, “do the rest”.
Clearly, this an attempt to encourage the remaining 360 user-base to migrate to the Xbox One rather than take their business elsewhere.
Moreover, though, it shines a massive spotlight on the lack of value for money offered by Sony’s Playstation Now streaming service, which is inferior on every conceivable level. Yes, emulating the PS3’s cell architecture is notoriously difficult, but that’s not really paying customers’ problem, is it?
A shrewd business move by Microsoft, and one that will undoubtedly bear fruit. An unexpected one, too, given the industry’s current penchant for last-gen HD remasters.
CORRECTION 21/6/15: Despite initial reports, it turns out that Xbox 360 games will actually all run using the same virtual machine solution. You can read more detail on it, and Digital Foundry’s preliminary verdict, here.