It would be nice to have out-of-the-box support for MiniTest::Unit for Ruby 1.8.x (in Ruby 1.9 Test::Unit is a thin wrapper around MiniTest::Unit, and for test cases inheriting after Test::Unit it works as usual).
Yes I found myself needing this today, and I think MiniTest::Unit should be fully extended in Ruby 1.9 as not everyone using the wrapper Test::Unit. This should be a simple fix though.
I don't use MiniTest so have no interesting in adding support for it, although I appreciate others would like this. I'm happy to look at and merge any work on implementing this.
I use MiniTest and do not use Webmock's TestUnit integration with it. I've been meaning to submit a patch for this, and the thing holding me back is how to test it.
Thanks to @phiggins, MiniTest support is now in master