Apparently, emulating the PS3's exotic hardware with the PS4's more traditional architecture would use up too much of the system's power and result in an even worse experience than just playing the things on your PS3. Sony intends to make PS3 titles playable "in some form" (likely related to their cloud-based game streaming).
While this is definitely some figurative snot in the wasabi, it was to be expected when they mentioned that the hardware wouldn't be directly backwards compatible anyway. If there's any sign of hope, it's that the statement refers to "current-generation" purchases. Some folks take that to mean that Sony's referring to the download versions of disc games and possibly games that are on the PS3 right now, in which case they'd naturally be incompatible. It's entirely possible that future games released on the PSN, following the PS4's launch, would be cross-platform compatible.
Whatever the case, don't junk or sell your PS3s just yet. You'll likely need 'em.
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