OK, figured it out.
I reworked the way how Instant UBB Code functions behave.
The buttons for Center, Superscript and Subscript were added and the behavior of some of the tags was changed.
Now you are able to highlight a word first, and then press one of the following tags: Bold, Italics, Underline, Quote, Code, Center, Superscript and Subscript.
These enhancements work in Internet Explorer, Firefox and Mozilla. They don't work in Konqueror, not sure about other browsers.
Check it out:http://www.shroomery.org/forums/post.html