Sacred is a loaded word full of cultural assumptions. What is 'sacred' is subjective. Some peoples think that everything and everywhere is 'sacred', but even then there are often 'special' places. Stonehenge is obviously 'special' but whether it has any ''religious' significance that is separate from other aspects of Stone Age culture is difficult to know. We have no idea how integrated the social, civic, cultural and arts aspects of Neolithic society were, although it is fair to assume that, given what we know about life at the time Stonehenge was built, there was a great deal of overlap, much as there is today in many societies (the state endorsed Christian Priesthood in the UK still wield immense political power in the House of Lords, for example)
We don't know precisely what was sacred to people in the Neolithic and Bronze age. We do know that Stonehenge was immensely important because of the effort and resources involved in building it and the precision of the design. It may have been a sky burial site, an observatory, a temple, a territorial statement or something else we can't conceive of. The people who built it chose an already 'special' landscape with many natural and unusual features that informed the positioning of their structures. The mythology of this landscape may have been extremely ancient already at the time of the build, and already lost in the mists of time, even then. We can be certain that it was a gathering place used by many thousands of people. Theories abound, some of which can be substantiated and are quite plausible, and lots of which are pretty potty, with no evidence or understanding of the site or its context.
We do know for certain, that whatever the intention of the original builders, it is certainly 'sacred' now, to the British in general (who own it collectively through English Heritage) and to Pagans especially. It is held sacred by the international community and this is 'certified' by its status as a World Heritage Site, which I think is a status denoting sacredness to all people/
We are hoping that the experimental archaeology being carried out by the Ancient Technology Centre will help to answer some questions about the lives of people who built it (see this thread for more info: http://www.druidry.org/board/dhp/viewtopic.php?f=167&t=41785