Progress update
The engineering team is still working on the implementation of this feature on multiple fronts: The cryptographic primitives are essentially finished, but there is still an effort required for finalizing the integration of crypto with consensus, e.g., for the X-Net re-sharing of a threshold ECDSA key, as well as some other parts of system integration. For example, the functionality related to the NNS proposals for key management (particularly Xnet re-sharing) still requires some more work to get finalized. A pretty small remaining item will be the integration with DFX which is essentially enabling the feature and generating / loading a key when DFX starts up a replica. Depending on performance measurements, we will need to give performance some attention, but that is currently open.
Another important task coming up is the security review of the consensus-related parts of the code through an external company. This will be started shortly and will help us increase our assurance on the code. We are planning to publish the resulting audit report once the findings have been addressed to share the gained insights with our community.
On the front of the closely-related Bitcoin feature, we have made great progress and will soon be able to run a first beta deployment on an IC subnet (Direct Integration with Bitcoin - #263 by dieter.sommer). So, the puzzle pieces are (slowly) coming together finally to enable a trustless Bitcoin integration unheard of so far on any blockchain!