There was a huge effort to make Moodle 1.8 comply with XHTML strict, and from now on, all new Moodle code should be valid XHTML strict, and any old code that isn't should be reported as a bug.
This was talked about at the developer meeting last week. If you have not seen them, there are some brief notes here: http://docs.moodle.org/en/Developer_conference_April_2007
Since the OU paid for all that accessibility work, I feel I should set a good example in this respect.