Update April 29, 2013: the barriers are still there, but nicely parted, no problem getting through.
I use Alpine Road on a regular basis to get to and from Skyline drive and El Monte Open Space.
But the county has put up barriers about 1/5 of the way down and 2/3 of the way up. I hear that there is absolutely no issue with the “road” (which is after all a trail), which makes this closure rather irritating.
The detour is a slog up a steep and poorly maintained (rutted) Coal Mine Ridge trail.