A year ago, the pipeline newsletter had an article "Improved synchronization across GPUs and CPUs " about the ARB_sync extension. That extension was never posted, nor (as far as I can tell) was anything to take its place, either as ARB_ or EXT_.
I realize one answer is “wait until OpenGL 3.0!”, but since that needs an approved specification, stable new drivers, and an application re-write, that’s a long ways off.
“ARB_sync” was started and developed against GL 2 (but not finished), then updated to incorporate the GL 3 object model, then folded into OpenGL 3 core proper. Now it is a simple matter of time and resources. Getting OpenGL 3 out has priority inside the ARB over all other matters. You’ll have to wait till OpenGL 3 to get improved fencing support, unfortunately.