I'm the Product Manager on the Core Server responsible for the multi-document transactions project. For those of you interested in learning more about how we're building transactions in MongoDB, I suggest checking out this video that discusses creating WiredTiger timestamps to enforce correctness in operation ordering across the storage layer. The description is presented by Dr. Michael Cahill, the co-founder of the WiredTiger storage engine aquired by MongoDB. https://www.mongodb.com/presentations/wiredtiger-timestamps-...
In 4.0, transactions will just be across replica-sets. The following release will have transactions across the entire sharded cluster (across multiple primaries).