This is getting rather silly; if you insert tags using Windows Live Writer it creates divs with a guid ID; which is invalid, as id and name attributes cannot start with a number. If you insert images it adds a contenteditable attribute which it appears to have up on its very own.
*sigh* Lack of attention to detail Microsoft.
[edit: 22-May 2007] After posting my whinge on the Writer forums I got a response from one of the team; the ID issue will be fixed in the release after next (the next release is in code freeze right now) and he'll look at the contenteditable problem as well; woohoo!