I agree with the other wise comments in this thread, but have one more point to add:

You can let your users use 'real' HTML tags (only those you allow, of course) instead of having them use other brackets or parentheses in an HTML-like language. Just update your regexps accordingly, it's really quite simple. Make sure you do filter out all non-allowed HTML tags, of course.

(When I saw the 'headline' on a Slashdot nodelet box, I just knew it was one of your threads :-) .)