After a bunch of fiddling and cross-browser testing I found that testing the existence (!==undefined) of properties on element.style worked well. So I put all of my rotation rules in a CSS class that’s applied only if the style properties exist.

<style type="text/css">
  h1.rotated {
    -webkit-transform-origin: left bottom;
    -moz-transform-origin: left bottom;
    -o-transform-origin: left bottom;
    transform-origin: left bottom;

    -webkit-transform: rotate(-90deg);
    -moz-transform: rotate(-90deg);
    -o-transform: rotate(-90deg);
    transform: rotate(-90deg);

    position: absolute;
    left: 0px;
    top: 0px;
    margin-top: 16px;
  }
</style>
<script type="text/javascript">
    // rotate the title, if that's even possible
    var h1 = document.getElementsByTagName('h1')[0];
    var s = document.body.style;

    // test for the presence of CSS3 transform properties
    if (s.transform !== undefined ||
        s.WebkitTransform !== undefined ||
        s.MozTransform !== undefined ||
        s.OTransform !== undefined) {
      // move
      h1.setAttribute('style',
          'top:' + ( h1.offsetWidth - h1.offsetHeight) + 'px');
      // and rotate
      h1.setAttribute('class', 'rotated');
    }
</script>

Of course, if I could just check for transform: support rather than -webkit-transform:, -moz-transform: and -o-transform: but we’re not there yet. Anyway, based on browsershots.org it seems to do the right thing on every known browser. It shows rotated text in very recent Webkit and Gecko browsers, and in Opera nightlies, and unrotated, correctly text everywhere else.