Writing and Technology
The Journal of Technology, Learning and Assessment has published a research review on the impact of technology on student writing. Analyzing 26 studies over the years 1992 to 2002, the research showed positive impact on the quantity and the quality of student writing. Another interesting aspect of the data was that the older the students were, the greater the benefit.
You can access the research at http://www.bc.edu/research/intasc/jtla/journal/v2n1.shtml. (Warning - high volumes of educational research jargon!)