Reiser4 has been in development going back to 2004 and had a promising future until the conviction of Hans Reiser and demise of his Namesys company following the murder of Reiser's wife, but since then its development has been stagnate. In late 2009 I was told by Edward Shishkin that Reiser4 may go for mainline inclusion in 2010. Before pushing again for mainline integration he wanted to publish an article in a scholarly magazine regarding Reiser4's design in order to facilitate some independent expert review.
The scholarly review and mainline push didn't happen. In January of this year I was then told by Shishkin that he was looking for vendor support to make the mainline acceptance much easier if there's some major Linux hardware/software vendor with interest in Reiser4.
Since January I hadn't heard any major news regarding Reiser4. The Reiser4 patches are hard to access as they haven't been re-uploaded to the Kernel.org infrastructure yet. However, Shishkin hasn't put out a Linux 3.0 patch for this experimental file-system yet due to addressing other bugs. The last time I benchmarked Reiser4 (against EXT4 and Btrfs) was January of 2010.
I reached out to Edward a few days ago for a status update and what his view now was on pushing for Reiser4's inclusion. He basically had to say that pushing Reiser4 upstream is not a high priority. Edward is not spending much time on Reiser4 any longer and in that time he's been spending it on bug-fixing. Below is the response I received back from Edward.
Pushing to upstream is still not of high priority for various reasons. I do have very restricted time for Reiser4 and spend it solely to address users complaints (there are old bugs in the code, which have become rather annoying).
Sadly it's looking less and less likely that we will ever see the Reiser4 file-system in the mainline Linux kernel. At least there's Btrfs...