On January 7, 2016 5:28:18 AM PST, Eric Lombrozo <elombrozo@gmail.com> wrote:

I am pleased to report that as of December 31, 2015 we have been successfully running a segregated witness testnet, called segnet, and have already implemented rudimentary wallets with support.

For source code, please look at sipa's github repo:
https://github.com/sipa/bitcoin/tree/segwit

And some example signing code at my repo:
https://github.com/CodeShark/BitcoinScriptExperiments/blob/master/src/signwitnesstx.cpp

Several wallets have already committed to supporting it including mSIGNA, GreenAddress, GreenBits, Blocktrail, and NBitcoin. More wallets are expected to be added to this list soon. If you're a wallet dev and are interested in developing and testing on segnet please contact me.

We're right on schedule and are very excited about the fundamental improvements to bitcoin that segwit will enable.

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Eric