It is true.
mod_rewrite can make it possible for search engines to spider dynamic sites and forums.
Most search engines will ignore pages with the arguments in the URI, but with mod_rewrite it is possible to make it appear as if they are all different pages.
An old CF CMS did a good job of this. All the pages were named ('1', '25', '3', '0', '0').html
To a search engine, this looked like a unique page, hoever each of those values were actually arugments passed by mod_rewrite back to the same page.
I have a CMS, phpx.org, and it uses a different method than mod_rewrite because not all servers have it.
If you would like to know more:
MOD_REWRITE Manual :
http://httpd.apache.org/docs/mod/mod_rewrite.html
Other:
http://www.stargeek.com/php-seo.php
http://www.phpbuilder.com/columns/tim20000526.php3
http://www.devarticles.com/c/a/Apach...h-mod-rewrite/