For months now Tom Stellard of the open-source AMD Linux graphics team has been working to get his R600g LLVM back-end merged into upstream LLVM. However, it has yet to be merged for different reasons and so it's been separately maintained in the Mesa tree until it's finally pulled -- hopefully by LLVM 3.2 but there still is no guarantee.
In hopes of easing the maintenance burden with handling an external LLVM back-end, Stellard has proposed a staging area where LLVM back-ends can live within the mainline SVN/Git tree until they're officially ready.
The point of the LLVM staging area would be to increase communication between upstream LLVM and back-end developers, ensure these new back-ends meet LLVM standards, and to increase the exposure of these up-and-coming back-ends.
For those interested in learning more about this staging proposal, see the LLVM development list message written by Stellard.