According to the source code https://github.com/mongodb/mongo/commit/a9e3511eda147d42f79203ad6e181e789c632c49 the number of split points was raised to 8192 which for default 64MB chunk size means 512GB maximum size for collection to be sharded.

Docs still say 256GB http://docs.mongodb.org/manual/reference/limits/#Sharding Existing Collection Data Size which should be 512GB (64MB x 8192 = 512GB)