I emailed Diigo about this in July and had a response from Joel Liu who said that they'll be adding it to the feature list (not sure whether this means that it's being implemented soon?). For the record my suggestion to Joel was:

".....My 2 cents on this is that email is often the easiest way of enabling content creation because most apps support email integration (in other words "send / share via email") and are often reluctant to add API's for every imaginable service that their users happen to be subscribed to.

I've found for instance that my use of Evernote (for notebook management) increased mainly because of their option to post via email. Another good example is Posterous (for blogging) where I can share photos from my iPhone or blackberry simply by sending a mail to post@posterous.com from within the native iPhone photo album. By allowing mailed submissions Posterous effectively extended their integration to each and every app that supports email sharing.

I already use the Mobile version of Post to diigo but if i'm scanning hundreds of Google Reader items or Twitter items daily then it would be much easier if I could just post to diigo via email."