WordPress MU

If you enter a CSS 'class' or 'id' declaration in a tag, it gets stripped during a Save or Publish. Why?

If it's a "feature" how can I disable it? I just spent the last hour looking through the wpmu files and couldn't find it.

Thanks in advance for any info on this.

This and other questions are answered in the FAQ I posted here:
http://donncha.wordpress.com/2005/09/23/wordpresscom-faq/

Excellent FAQ. Thank you!