I think you're not the first one trying to make autotest work with steak. Personally I never tried to do so and I don't know if it is possible out-of-the-box or if specific support from steak itself is necessary. All I can do is pointing out that ultimately a steak feature is just a rspec spec, so if there's any standard way to autorun regular specs (autospec?), that should also work with steak specs.
That said and despite cucumber supports it, I don't see much point in running acceptance tests with a tool like autotest. Acceptance test suites tend to be much more slower than unit test suites, so as your project grows, soon it becomes completely unpractical to autorun the whole suite every time you make a change in your application code. (Again, I've never been a user of autotest with cucumber, so please correct me if any of my assumptions are wrong)
For cucumber, there's a clever tool called cucover[1] that might help with this problem. It runs a feature only if it needs to be run based on the changes in the source files "covered" by the previous run of that feature. If such a tool would exist for rspec (and so, for steak) and if it'd work well, then running acceptance specs with autotest might make sense to me. Anyone interested in writing it? I would help.
[1] http://github.com/mattwynne/cucover
-- Luismi