If using PPT 2007, you may have found the super and subscript buttons missing from the ribbon. My first solution is to make use of the QAT (Quick Access Toolbar) and add these valuable tools to it.

But you can also make any text superscripted with a keyboard command.

This works in PPT 2007, 2003, XP, and 2000 (cannot remember if works in PPT 2000, so this is just a guess).

A few notes about using the keyboard options from my use and observations:
1. PPT 2003 CTR+SHIFT+(plus) superscripts. Do it again and it returns to
standard text.
2. PPT 2007 CTR+SHIFT+(plus) superscripts. Do it again and nothing happens
(eg. no way to return to standard text from keyboard commands).
3. I have never been able to use keyboard command in either version to sub-script text (although others have said it works).