Coming to US on Acorn TV on 16 March 2015
Acorn TV (subscription streaming service) will offer Jamaica Inn starting on Monday 16 March, adding eps two and three on the ensuing Mondays (one per week). After an episode premieres, it will remain in the Acorn TV online library for a while, though I'm not sure how long. (Some of the other content has been available for months, so Jamaica Inn could remain for quite some time.)
So look for Jamaica Inn eps on March 16, 23 and 30.
http://acorn.tv/comingSoon
Acorn TV also offers a 30-day free trial.