XHTML error within a post – any advice?

This is more of an XHTML question, but I think the cause may be a WordPress issue.
Basically, I’ve been trying to keep my site XHTML valid, however for some reason my latest post doesn’t validate. I put a < div > inside my post to center some images and text, however I keep getting a end tag for “p” omitted, but OMITTAG NO was specified error for the line containing the closing < /div >. I’ve been fiddling around with it, but I can’t seem to correct the problem. I even tried sticking in a closing < /p > tag, but WP simply removes it.
I’d really appreciate it if someone could take a look at the errors and source code (here) and give some idea of what I’ve done wrong and how to fix it.
Thank you advance!