Carnegie Mellon and Penn State support several related projects. Some names associated with this topic are Kurland, Trigg, Davida Charney and Chris Neuwirth. Basically, as you might suspect, there seem to be good and bad effects associated with using computers. However, if you just want to get people to practice writing, they do tend to write more. The down side is that people get caught up in activities like formatting and font styles which are peripheral to the goal of writing better.
Some systems designed to support particular tasks can be quite useful.
For instance, there are some hypertext tools which guide novice writers through heuristic activities supporting critical thinking and analysis which seem to be effective sometimes. Practice is a very significant factor. The impact of computer-based tools and embedded prompts on writing processes and products of novice and advanced college writers. Educational technology, research and development. Conference Proceedings Distance Learning papers presented at the 1996 AERA Conference Lane, Carla.