http://www.dorsetsafetycameras.org.uk/
The front page has a link (updated on mondays) to a list of the week's mobile sites. This week includes A37 Holywell Cross.
Last week (20th September to 26th September 2005) they were doing A37 Wardon Hill and A37 Long Cross, Shaftesbury
The 2 weeks before that (31st August to 12th September 2005) they were doing A37 Long Ash Lane and A37 Staggs Folly.
If they wanted to slow people down they'd put up proper warning signs and make the camera visible.
I just happened to find a picture of the portable gatso in use on the A37:
http://www.speedcam.pwp.blueyonder.co.uk/van96.jpg
It's the little black box almost dead center in the photo under the sign. Highly Visible my bottom.
There's some better pictures on the web site I found that on, here:
http://www.speedcam.pwp.blueyonder.co.uk/van3.htm (note the yellow one used in Essex at the top...)