The closest thing I've seen is an addin from John Walkenbach:
http://j-walk.com/ss/excel/files/supersub.htm

Neight wrote:

Is there a shortcut key to switch to subscript and switch back while typing?
It's a pain to have to highlight just a couple of charactors in each of the
multiple cells I have to have a subscript in. Can I assign a shortcut key?

--

Dave Peterson