I like this suggestion as well! I have a small community and only send out a newsletter three times a year, but it would be great to have this integration.
Currently I import my email addresses from my forums and add them to my MailChimp listing making note of the users that don't want to get emails from me. It's a round about process that I would greatly love to reduce!
I was planning to write this myself if nobody else does.
The old VB-Mailchimp plugin makes use of custom user profile fields and XF currently do not support it. So it all depends on if the final XF will have user profile fields.
I think that mailchimp/aweber are NOT a good solution for forum newsletter...They are builded for professional infomarketers and the DEM market, for persons that want a professional use of e-mail marketing. For a medium/large forum it's just too expensive.