Hi
there has been more studies done on activPAL. I use it both clinically and for research. I am currently validating it for neurological population.
I have out an article that give some information about activity monitors on PhysioToday - Care Learn Talk - home
but check out these references
Grant P.M., Ryan C.G., Granat M.H. (2006) The validation of a novel activity monitor in the measurment of posture and motion during everyday activities. British Journal of Sports Medicine 2006;40:992-997
Ryan C., Grant M.P, Tigbe W.W., Granat M.H. (2006) The validity of a novel activity monitor as a measure of walking. Br J Sports Med. Published Online First: 6 July 2006. doi:10.1136/bjsm.2006.027276

GIve me a shout if you need copies I have electroic versions.

Cheers