After seeing this post I asked my sales rep about this.
The story is that ISE 2.0 is tentatively on the road map to be announced next year. At that time TACACS+ support will be included and should have feature parity with ACS.
ACS may go EOSale at that time, with EOL/EOSupport date being 3-5 years after that. Of course the usual pricing incentives to encourage migration will probably be available. I'm also guessing that there will be some kind of migration tool for the config available as well.
Bottom line: I don't feel like my investment in ACS (3 servers, over 500 devices each) is a waste. It's still a viable product for more than a normal product lifetime.