I know that Yrjo, too, has urged such differentiation, separating the
genesis of activities as between the ontogenetic and cultural-historical
level. I tend to think of activity systems as cultural-historcial level
phenomena, but obviously artifact mediation is implicated at all levels,
and thus, so is artifact-o-genesis.
The article by Griffin et al in the Context of Learning book has a good treatment
of computer sortware as a participant in the teaching learning activities it
mediates.
mike