"The Secretary of State has called in the application [on the proposed new visitor centre] because she "considers that the proposals raise issues of more than local importance having regard to their relationship both to the Stonehenge World Heritage Site and to the Government's proposals for upgrading the A303." "
http://www.24dash.com/content/news/viewNews.php?navID=2&newsID=9748

The point being, the last risk that the short tunnel could be scuppered by locals refusing the visitors centre (which is said to be central to the scheme) has been removed as the final decision has been taken away from them by central government.

That, plus the highly peculiar scare story that was put about recently that Stonehenge might lose its world heritage status unless the government sorted out the roads situation points to the fact we really are going to have a "short tunnel" decision announced (together with the integral part they never mention - its huge surface entrance cuttings across one third of the World Heritage area).