this great article on composer and musical creativity researcher David Cope (here's a 2009 NPR interview with him). Cope is a composer who has spent many years developing software to help him explore and understand musical creativity. The software he developed (EMI or Emmy, and his newest creation, Emily Howell) actually composes music. Cope's basic hypothesis on musical creativity is that it is fundamentally imitation and redeployment of music the composer has heard before. His first program (EMI, mp3 samples here) was based on the analysis of many pieces of music to extract what you might call the style of the original composer, though it really goes deeper than style to include sources and influences. EMI (Experiments in Musical Intelligence) could then create new pieces of music in that style (or combination of styles), music that was often considered by experts to be of high quality and practically indistinguishable from music by the original composer - until they were told that the composer was a computer, after which they judged the music to be derivative and mechanical, as well as relatively uninteresting since they are new works written in old styles (e.g., Bach).
Emily Howell in some ways seems like a new interactive "front end" for EMI. Using it, Cope communicates musical ideas and goals. The software responds to these goals with modifications to the music "under construction" (it uses the output from EMI as its musical source material). In this way, composition becomes much more of a collaborative process between Cope and Emily Howell, and the results are rather impressive, based on the samples I have heard, with a sound more like modern music than re-fried classics. Emily Howell's first album is supposed to be released soon on Centaur Records.
As a songwriter (but definitely not a "composer"), I find this cool and exciting. It's really just taking musical tools to a new level. In their early days, synthesizers and sequencers were considered to be threats to studio musicians and orchestral players, but they have truly expanded the palette of tools that musicians have available (though I'm sure some musical jobs have been lost or displaced in the process, while others have been created). I write most of my songs with a guitar or keyboard and record most of them with the help of a talented (human) producer and musician who can arrange things I can't arrange and play things I can't play. In addition to recording software (SONAR), I sometimes make use of a program called Band-in-a-Box which generates fairly sophisticated automatic accompaniment through MIDI-based synthesizer voices. These are great for demos and they have inspired some of my best songs by allowing me to easily play around with different styles,arrangements, and instruments. I'd be happy to work with a smarter version of BIAB (like Emily Howell) if it becomes available at an affordable price.