I appreciate having in one place my reference materials for my projects. I would love to have in doit.im the documents I need to read or use in a meeting.

This would mean that doit.im would allow me to attach a document to a task and/or project. It also means that any emails with attachments I send to doit.im to create a task would have their attachments included on the created task.

I think the largest problems to solve would be where we/doit.im stores attachments and can the Pro fees cover the additional cost of storage.

Here is how other services handle document/attachment storage. All of these accept attachments via emailed tasks.

Get It Done: user's documents remain on the server, links in mobile and Web apps. US$39/yr

Toodledo: user's documents remain on the server. Mobile apps (third party) do not handle the attachments. US$30 per year.

Evernote: not strictly a task/project management tool. Optionally sync documents (and notes) to mobile devices. US$45 per year.