I know i've missed the boat slightly with this one but have just joined the site and have a few things to add. As a local to the preseli area I have often wandered about the blue stones going to stonehenge. Several programes I've watched about stonehenge mention the fact that of the so called "blue stones" only about half of them are spotted dollerite and actually came from carn meini. The rest are a mixture of stones believed to come from south wales generally.
On this note I came across an excavation in the valley bellow my house last autumn, Mike Parker Pierce (I think), renowned stonehenge archaeologist and a team from sheffield university were excavating around a rock outcrop on the avon berian valley, a tributary of the river nevern. Geologists had taken samples from the outcrop (looking for the sources of the other "blue stones") and the end of the outcrop had given them a exact match for 2 of these stones, 5 or more other samples had been taken from the outcrop, but the sheer cliff side was the only true match. They'd excavated along the side of this cliff face and found a couple of stone tools, they then came out further from the face and unearthed a stone some 10 foot by 4 foot by about 2 foot thick. They were convinced that this was a neolithic quarried stone destined for stonehenge or some local henge, having been abandoned because it had cracked and a section underneath had come loose.
The theory that the blue stones were carried to salisbry plain by glacial action can't really account for this, a neolithic quarry site in a enclosed valley, on the wrong side of the preselis with an exact geological match for stones that are part of stonehenge.
The archaeologists are due back at some point this summer to take the excavations further, looking for evidence of which way the stones were moved (up stream towards the preselis or down stream towards the sea)... They were very excited and said that this was the first time a neolithic quarry had been found with an abandoned stone. Funnily enough at their talk they did in Newport after the dig, Brian John stood up and did his "we all know it was glaciers" speech, but I just don't see how they'd take a few stones from one place, a couple more from somewhere else and them all be the right size to be used in a structure like stonehenge