file-system has been ported to the Linux 3.5 kernel.
While Reiser4 isn't mainline in the Linux kernel tree, it can at least now be patched to work on the Linux 3.5 kernel. Previously this controversial file-system would work on the Linux 2.6.39 kernel, then ported to Linux 3.2, and now it's wound up compatible with the Linux 3.5 kernel
More details can be found from the SourceForge patch area
Back in May, it was deemed not a priority to port Reiser4 to new kernels
(or to focus on getting it mainline) but rather to push the limited resources towards getting bugs fixed.