Box.net has added iSpeech to its OpenBox platform, which lets users get integrated text-to-speech conversion on any text document they have stored on the service. Users simply have to add it to their list of used OpenBox services and it becomes a part of Box.net's contextual menus, meaning you only need to right click on the document and choose the text-to-speech option to get it going.
You do have to be separately signed up with iSpeech to get this to work. It's not a free service when it comes to processing full length documents. There is, however a free tier of service that gives you 250 words per conversion, which amounts to a couple of paragraphs. If you feel like converting your doctoral thesis you'll need one of the service's premium plans.
While neat, I think a far more useful add-on to Box, or any other storage provider would be converting audio recordings into text documents. I've recently become hooked on this with my voicemail through Google Voice, and it would be great get a recording from an interview, or business meeting transcribed in the same place I'm storing it in the cloud.