23 December 2008 MerbAndRailsMerge see http://yehudakatz.com/2008/12/23/rails-and-merb-merge/
And as of a couple days ago (early September, 2010), the merge was published as RoR Version 3.
Rails's dirty little secret was its gleaming awesome programming interface hid a core held together with spit. Merb has a truly awesome core built entirely to be pluggable, so I would expect the merge consisted of building Rails programming interfaces as pluggable modules and then plugging them in...