You should be able to support all the way back to IE6 and Blackberry, although you need to include a few polyfills ( http://lhorie.github.io/mithril/tools.html )

Re: TodoMVC - there was someone working on an implementation here: https://github.com/jpmonette/todomvc-mithril

I think it's not fully implemented yet, but you should be able to see the general look and feel of the code.