http://www.bloomberg.com/news/articles/2015-08-04/wall-street-meet-block-368396-the-possible-future-of-finance provides a hint: block 368396.

When looking through that block, there are three notable transactions, with bare multisig encoding:

- https://blockchain.info/tx/adb8129bed9efb0b6ebf4bac8fee2d70ab41fea7a0027b9371c100fa360bb38a
- https://blockchain.info/tx/1c751d4af1fa2385d6df008fede770f365b71c8138b180cc75813e0955d3d0ea
- https://blockchain.info/tx/b7547019ee64f438a05d4458ff879765d984a541a046871d5495d0d0cf92c413

Looks like the Symbiont mainnet testing started at least two days ago.