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I guess it's up to you to determine the semantics, but in my opinion:
Rather than a definition list, form-related properties should be used.
<form>
<label for="fullname">Full Name:</label>
<input type="text" name="fullname" id="fullname">
<label for="email">Email Address:</label>
<input type="text" name="email" id="email">
</form>
The "for" attribute in the <label> tag should reference the "id" attribute of a <input> tag. Note that when labels are associated with fields, clicking the label will put the associated field in focus.
You can also use tags like <fieldset> to cluster sections of a form together and <legend> to caption a fieldset.
I've successfully used the technique outlined in this article several times.
I agree with sjstrutt that you should use form related tags like label and form in you forms, but the HTML outlined in his example, will often lack some code you can use as "hooks" for styling your form with CSS.
As a consequence of this I markup my forms like this:
<form name="LoginForm" action="thispage">
<fieldset>
<legend>Form header</legend>
<ul>
<li>
<label for="UserName">Username: </label>
<input id="UserName" name="UserName" type="text" />
</li>
<li>
<label for="Password">Password: </label>
<input id="Password" name="Password" type="text" />
</li>
</ul>
</fieldset>
<fieldset class="buttons">
<input class="submit" type="submit" value="Login" />
</fieldset>
</form>
This approach leaves me with a comprehensible set of tags, which contains enough hooks to style the forms in a lot of different ways.