Is there any way to have a fieldset width only be as wide as the controls in them?

Use display: inline-block, though you need to wrap it inside a DIV to keep it from actually displaying inline. Tested in Safari.

<style type="text/css">
    .fieldset-auto-width {
         display: inline-block;
    }
</style>
<div>
  <fieldset class="fieldset-auto-width">
      <legend>Blah</legend>
      ...
  </fieldset>
</div>

fieldset {display:inline} or fieldset {display:inline-block}

If you want to separate two fieldsets vertically, use a single <br/> between them. This is semantically correct and no harder than it has to be.

You could float it, then it will only be as wide as its contents, but you'll have to make sure you clear those floats.