How can I replace the "Add to cart" button with an "Ask about product" button, based on product property?
I figured out answer, and it's not that bad as I though. I decided to write a tutorial for it as it looks like very useful feature. So here it is:

Tutorial


Install everything, I mean, Commerce and Webforms,
Create example webform - let's name it "Contact",
Create Product Display, create some product with few attributes, and one boolean field (not attribute) "Contact required",
Configure webform, specifically, create there a field that will have default value, fetched from GET query string (how to do this: http://drupal.org/node/296453#webform-url-default ), we will pass to our Contact form a SKU of our product by using GET method.


Now create your own module, I will not elaborate on that, as there is a lot of info out there. The hooks that we will need are:


  HOOK_form_commerce_cart_add_to_cart_form_alter(&amp;$form, &amp;$form_state)
  
  HOOK_commerce_cart_attributes_refresh_alter(&amp;$commands, $form, $form_state)


The first one will allow us to manipulate our form initially (if default product will have "Contact required = true"), the latter will allow us to adjust our form to AJAX calls.

Yes, that means that, our "Add to cart" button will be dynamically changing based on current product "Contact required" field. Looks promising doesn't it? :)

We will also use two helper functions that are not hooks: 


  MODULE_ask_for_product($product)
  
  MODULE_contact_required_validate($form, &amp;$form_state)


The first one will return HTML of our new button "Ask for product" (I encapsulated it in function, because we will need that data in both above hooks),
and the second one will validate our form, so if "Contact require" field on product, will be set to 1, the form will not validate, and product will not be added to cart. The submit button for regular user will be removed, but this is only against smart guys that would like to annoy you by sending this form even if you hide submit button.

Those are the functions:

    function HOOK_form_commerce_cart_add_to_cart_form_alter(&amp;$form, &amp;$form_state) {
    // Adding new class to our button, that will allow us, to target it by jQuery
    $form['submit']['#attributes']['class'][] = 'add-to-cart-button';
    // Adding new validator, that will make sure that our products with required contact, will not be added to cart
    $form['#validate'][] = 'MODULE_is_contact_required_validate';

    // Getting value of field_is_contact_required of default product
    $product = $form_state['default_product'];
    $field = field_get_items('commerce_product', $product, 'field_is_contact_required');

    // If value is 1, then replace our button with markup.
    if($field[0]['value'] == 1) {
        unset($form['submit']);
        $form['ask_for_product'] = array(
            '#markup' =&gt; MODULE_ask_for_product($product),
            '#weight' =&gt; 50,
        );
    }
    }


Now we need to do something with AJAX:

    function HOOK_commerce_cart_attributes_refresh_alter(&amp;$commands, $form, $form_state) {

    if (!empty($form_state['context'])) {
        // Getting value of field_is_contact_required of new chosen product
        $product = $form_state['default_product'];
        $field = field_get_items('commerce_product', $product, 'field_is_contact_required');

        // If field_is_contact_required is set to 1, then replace a container that have class ".add-to-cart-button" (we just added that class), with our markup
        if($field[0]['value'] == 1) {
            $replacement = '.add-to-cart-button';
            $commands[] = ajax_command_replace($replacement, MODULE_ask_for_product($product));
        }

    }

    }


And our helpers:

// Return markup of our new button. 'node/7' is node of our contact form, 'subject' is a key of our query string
function MODULE_ask_for_product($product) {
    return '&lt;div class="ask-for-product"&gt;' . l(t('Ask about product.'), drupal_get_path_alias('node/7'), array('query' =&gt; array('subject' =&gt; 'Ask about product with SKU: ' . $product-&gt;sku))) . '&lt;/div&gt;';
}



// Prevent some products from be added to cart.
function MODULE_contact_required_validate($form, &amp;$form_state) {

    $product = $form_state['default_product'];
    $field = field_get_items('commerce_product', $product, 'field_is_contact_required');

    if($field[0]['value'] == 1) {
        form_set_error('', t('Don\'t try to be annoying! :P'));
    }

}


That's all, you can customize it, or by extending the idea, do much more with it (maybe by using webform api).