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.
Am I missing some other extension spec?