If you want to use Bookmarklets and Movabletype while using a superior browser like OmniWeb or Safari, you have to alter the bookmarklet code slightly.
There were two entries in Google when I searched "Post to MT Weblog" and OmniWeb. One was a post to this site complaining about it and the other was to the solution at Jay Allen's immensely wonderful website -- also of MT Blacklist fame -- the bane of all spam comments.