I think the link being on this page says a lot about the movement: http://www.roundcube.net/?p=forum. Its authenticity cannot be denied as it's listed at the top of the page. I think in order to gain an idea of the scope of the movement we need to ask a different question: do people intend to continue using the mailing list as actively as before the forums sprung up?
I've noticed that the mailing list is fully moderated, (please correct me if I'm wrong). Sometimes posts do not show up for a few hours if at all. I made a post to this list recently that never appeared. I've noticed that some days, there are no posts to the list at all and then a flood of them pours in the next day as the moderator was able to find the time to approve everything. In a forum, while moderated, posts show up immediately and can only be removed as someone with moderation privileges finds the time to check the forums.
There are obviously heavy pros and cons on both forums and mailing list. It is nice to be able to direct a mail to the general community and the core developers and expect that those people will at least see the subject line in their email headers. A post made to a forum could be missed as it cascades down the page for a couple of days. Intuition can only imply that the mailing list will remain an integral part in the communication of the development community and user base as long as it continues to be hosted.