I made a form in my PHP / jQuery Mobile web application with this HTML.
<form action="<?= $_SERVER['REQUEST_URI'] ?>">
<!-- data received from a previously submitted form -->
<input type="hidden" name="firstName" value="<?= $firstName ?>" />
<input type="hidden" name="lastName" value="<?= $lastName ?>" />
<div data-role="fieldcontain">
<label for="favoriteWord">Favorite Word:</label>
<input type="text" name="favoriteWord" id="favoriteWord" value="" placeholder="Please tell us your favorite word." />
</div>
<input type="submit" name="submit" value="Submit Form" />
</form>
However, my submit button looks odd. It looks overlaid on itself (as my screen shot shows).

Why?
This is the HTML I see when I do use data-ajax="false" (The problem still persists with ajax off) and hit "View Source".
<form action="/submitFavoriteWord">
<!-- data received from a previously submitted form -->
<input type="hidden" name="firstName" value="David" />
<input type="hidden" name="lastName" value="Faux" />
<div data-role="fieldcontain">
<label for="favoriteWord">Favorite Word:</label>
<input type="text" name="favoriteWord" id="favoriteWord" value="" placeholder="Please tell us your favorite word." />
</div>
<input type="submit" name="submit" value="Submit Form" />
</form>
Update: I analyzed the page in Chrome Web Developer. Here is what the Elements tab suggests jQuery Mobile is generating.

Strangely, there is a <span> tag generated above my submit button.
This example seems to work fine: http://jsfiddle.net/shanabus/FG97U/
So maybe its a problem with an unclosed HTML element somewhere else on the page? This can be verified if you post more of your code, but I have seen this happen and checking the other HTML on the page may solve your problem.