iRite or HTML Editor?
irite uses an old version of FCKEditor that does not support tab indexes.
if you are using HTMLEditor, it uses a newer CKEditor which does support setting the tab index.
find the following line:
[$CKEditor_config["tabSpaces"] = 4;
and change it to:
$CKEditor_config["tabSpaces"] = 4;
$CKEditor_config["tabIndex"] = 2;
HTML Editor is rich text editor though, and tabbing is valid input, so you wont be able to tab out of the HTML editor box, you will need to manually select the next form element using a mouse click.