The release with TACACS support will be 2.0. Beta testing is kicking off starting in July.
The release date won't be committed until the beta is well underway but this fall (2015) would be a safe bet.
Not sure if its open beta program or closed, if open what is the process to join the same, would be very much interested in trying my hands on it and be part of beta testing.
The beta is not open - Cisco has an internal nomination process via the Security Consulting Systems Engineers (CSEs) and/or ISE ATP Partners.
The call for nominees just went out late last week so if you contact your local SE you may be able to get them to put your name in for consideration.
Which TACACS+ features will not be supported in ISE 2.0 that do exist in ACS 5.x today?
I would like to decommission our ACS deployment and fully use the ISE in the future.
do you know if there will be separate licensing in ISE 1.5 for TACACS device management, or if it will rely on the existing licensing model. With current model it would count # of concurrently logged in network admins into equipment, which is a little strange.
ISE 2.0 has been released with 3rd party support and TACACS+
See the new feature documentation for scale and any ACS>ISE Caveats