peeper.js lets you bind dom elements to text inputs with one simple assignment: data-source="[input id]"

First make sure to include the library along with jquery:

<script src="jquery.js" type="text/javascript"></script>
<script src="peeper.js" type="text/javascript"></script>

Then simply assign the id of the watched input to the peeper's data-source attribute.

<input type="text" id="observed_field" />
<span data-source="observed_field">placeholder text</span>