The DVD would only be able to read the DVD layers ie. only access 8.5 gigs of data.
The question is whether a blu-ray player (that can play both blu-ray and dvd) will go insane trying to decide which format to read when faced with both at once, PS3 could fix this with a firmware update but all the standalone players might be screwed if this is fully implemented. Or they may have a preference for the DVD layer and might make all movies of this type play only in DVD mode and not Blu-ray mode.
So i kind of laugh at the backwards compatible notion.
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