http://www.groovecommerce.com/ecommerce-blog/ecommerce-web-site-design/creating-an-accessible-checkout-progress-bar/#comment-11644
An interesting article on how this could be done. Due to the reasons below I would probably make the following modifications:
1. To satisfy DDA requirements, information that is conveyed by an image (e.g. the progress bar image) also needs to be conveyed by screen-reader accessible text on the page. For step 3 from your example page I would implement it as below; inserting " (selected)" after the step name provided within the <span> tags.
<h3>Step 3</h3>
<ol title="Checkout Progress" id="step3" class="checkout-progress">
<li>
Checkout</li>
<li>
Billing and Shipping</li>
<li><span>Confirm Your Order (selected)</span></li>
<li><span>Complete Order</span></li>
</ol>
2. When I last researched the issue, most screen-readers do not come pre-configured to read out the title text of a link. It is entirely possible that this situation has changed, but if this is still the case I would be inclined to further modify the example by placing the "Return to" text inside <li><span> instead of using title text. I would also be consider using "Back to" and "Forward to", rather than "Return to" as these re-enforce the ordering by indicating the direction through the process that the user would be moving. Further modifying the above example with the steps above and to add a forward link for the final step this would become:
<h3>Step 3</h3>
<ol title="Checkout Progress" id="step3" class="checkout-progress">
<li>
Back to Checkout</li>
<li>
Back to Billing and Shipping</li>
<li><span>Confirm Your Order (selected)</span></li>
<li>
Forward to Complete Order</li>
</ol>
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