Well then. How do you display text as a fraction, a superscript or a subscript in Inventor? Well Inventor takes a few leafs out of the venerable AutoCAD book in this respect. Once you're inside the Text Editor dialogue box (access this with the 'pencil' button if you're editing a dimension) it's a simple use of the 'stack' button that you're after. But you gotta know the right symbols!
The trick is to highlight the text you want to format and then hit the 'stack' button.
Here's my little summary of simple formatting options. Note that a carat '^' symbol at the start of the highlighted text will produce a subscript, and a '^' at the end will produce a superscript. Enjoy!