Attach jQuery behavior to AJAX loaded content
Ok I managed to make it work.

Drupal did try to re-attach the behavior on my content after it was added by AJAX, but my selectors were starting from too "early" in my DOM - as such, when drupal.attachBehavior got called by the system, it couldn't find the right selector.

Example :
My DOM look like this :

&lt;div id="product-add-to-cart"&gt;
    &lt;div class="class1"&gt;
        &lt;input ... class="input1" /&gt;
        &lt;input ... class="input2" /&gt;
        &lt;input ... class="input3" /&gt;
    &lt;/div&gt;
&lt;/div&gt;


My jQuery selector looked like this
    $('#product-add-to-cart .class1 input.input1").hover(...);

As I understand, FormAPI was replacing what was INSIDE #product-add-to-cart .class1, while the context passed to Drupal.attachBehavior only had part of this markup (the input elements..) : hence    $('#product-add-to-cart .class1 input.input1").hover(...); couldn't return anything in this context.

I replaced my jQuery $('#product-add-to-cart .class1 input.input1").hover(…); with 
$('input.input1").hover(…); and it works like a charm.