As written about in an earlier Phoronix article in detail, "DRI3" was proposed during the development event last week. Keith Packard and Eric Anholt came up with some ideas while consuming some tasty Franconian beverages for improving DRI. There's also the video of the XDC2012 updated DRI presentation.
The problems they acknowledge with DRI2 is that there's potentially a damaged/torn frame when resizing an X window, no information about back-buffer contexts, unpurgable stale buffers, driver-specific buffers, and GEM flink has a lot of issues.
The proposed changes for DRI would come down to eliminating DRI/DRM magic-cookie based authentication, allocate all buffers in the application, present buffers through DMA-BUF, changing how buffer allocations are managed, notifying applocations of idle buffers, and releasing buffer memory.
At this time it's not known whether these DRI improvements would materialize as "DRI3" or would be left as DRI2, thus Keith is referring to it as "DRI-Next" for the time being.
The new DRI plans are formally outlined on Keith's blog.