I stumbled across a cool little service at by-voice.com. Basically, this is a simple web-service by which you can get text converted to computer-generated speech. The service works by making a HTTP request to by-voice, which means its a little awkward for everyday users, but it's wonderful for programmers, because you could, if you were to sign up to the service fully, write programs that would on-the-fly download speech and play it.