It's estimated to be 10m wide and runs down to the River Avon from the Durrington Walls henge (which is larger than Avebury, by the way, but without the standing stones).
This is a major find, and apparently unique. The trackway is metalled with flint cobbles and the entire area is strewn with broken pig bones, evidence of long term habitation and an extensive midden.
Evidence has also been found of the henge itself having being built in sections, perhaps with different work gangs working alongside each other, or the same gang working around the perimeter in stages.
Red deer antlers used as pickaxes to pry the chalk from the ground have been uncovered and the dating evidence shows the area to have been in use at least as early at the third stage of the construction of Stonehenge (the erection of the sarsens).
Channel 4 will be showing a TimeTeam programme about it
next April on November 28th 2005 @ 9pm, and the preliminary site survey results are expected to be published towards the end of this year.
In addition, Prof. Clive Ruggles has been researching possible archaeoastronomical alignments in the surrounding landscape, with particular attention to those places on the Stonehenge solstice axis.
They had an open weekend last weekend, and so we went down for the tour - it's rare for permission to be granted to dig in the Stonehenge World Heritage site, and the excitement from the academics and students at what was coming out of the ground was obvious.
Updated: There's a discussion forum where some of the archaeologists from the dig have posted a few first hand accounts.
The background to the project, and PDF interim report.