This deserves a post to itself, despite my intention to reclaim this blog for its core purpose. I don’t have a considered reaction to this news, so I won’t write a detailed response. The one thing I’ll say is that it does appear to be a fairly direct result of the boycott. (Edit: Alicia Wise, Elsevier’s “director of universal access” denies this.)
Elsevier’s (slightly grudging) announcement is here.
Edit. Elsevier announce further concessions here in a letter to the mathematics community.