What is this mystical force that causes the answer to reveal itself as soon as you post a question somewhere?
It looks like in MVC 3 you disable client-side validation on a button by adding the class “cancel” to it. So in my example:
<input type="submit" name="backButton" value="← Back"
title="Go back to step 1." class="cancel" />
works great. And no ID attribute is needed either. If you have an actual style class on the button, just do this:
<input type="submit" name="backButton" value="← Back"
title="Go back to step 1." class="style-name cancel" />