If you are defining your own <input>
for the filterable object, just define a placeholder right on the input:
<form class="ui-filterable">
<input id="filterBasic-input" data-type="search" placeholder="find cars" />
</form>
<ul data-role="listview" data-filter="true" data-input="#filterBasic-input">
<li>Acura</li>
<li>Audi</li>
<li>BMW</li>
<li>Cadillac</li>
<li>Ferrari</li>
</ul>