The symmetrize operator is now in trunk, r51356.
Since the 2.64 release, I’ve been getting a few of the commits from the dyntopo branch finalized and into trunk. The symmetrize operator is a nice independent piece of code from dyntopo, much as the convex hull operator was an independent piece of the skin modifier.
Note that during the code review for symmetrize some fixes were made; hopefully it should work better with dyntopo now. In particular, there was a bug that caused small unattached faces to hang around the plane of symmetry. That should be fixed now.
Thanks to Brecht for reviewing the code.
If you find any bugs, please report them in the regular Blender bug tracker.
Development continues on dyntopo as well. Most of the work is still related to the undo system, but there have been other stability fixes and code cleanups as well. Of note is that the layer brush no longer crashes.