I believe faranth is using the Enhanced (WYSIWYG) Editor and not the standard one like myself and Snake~eyes which is why he doesn't get any problems.
Seems to be something todo with the naming of this text area, the error I get is:
Which points to theform.message.focus(); (in the following function in vbulletin_stdedit.js)
Code:
// #############################################################################
function setfocus()
{
theform.message.focus();
}