p vs. ol or ul for form styling

In my opinion a group of form controls is neither a list item or a paragraph. When I mark up forms, I separate my groups of label/input by wrapping them in <div>s. If you're trying to mark up a division between form controls, then don't be afraid of using a <div>.

<fieldset>
  <div class="field">
    <label for="txtName">Name</label>
    <input type="text" id="txtName" />
  </div>
  <div class="field">
    <label for="txtTitle">Title</label>
    <input type="text" id="txtTitle" />
  </div>
</fieldset>

From your given examples, <p> probably degrades "better" because you won't see bullets next to your items if CSS was unavailable, and the groups of controls would probably be spaced out fairly well.

Don't forget Definition List:

<fieldset>
    <dl>
        <dt><label for="txtName">Name</label></dt>
        <dd><input type="text" id="txtName" /></dd>
    </dl>
</fieldset>

There's no real right answer. Pick the markup that makes the most sense to you. To me, forms aren't an unordered list. Definition list is closer in my mind.