JR East released a whole slew of updates today: in addition to the Tohoku Shinkansen as covered by jib71, almost all services in Tohoku are expected to be back and running by the end of April, with the exception of chunks of the Tohoku Main Line (which can for most part be bypassed by the Shinkansen) and some coastal lines in the directly tsunami-hit area. Even eg. Kesennuma should be back on the map by April 10th.