Damn, knockout.js 1.3 Control Flow is handsome.

gistfile1.html

<div id='Page'>

  <div id='Side'>

    <div id='Header'>

      <!-- ko with: commandline -->
      <input type="text" data-bind='value: command' />
      <!-- /ko -->

    </div>

    <!-- ko with: clients -->

      <ul id='Clients'>
        <!-- ko foreach: items -->
          <li data-bind='click: select, css: {selected:selected}'>
            <span data-bind='text: data.name'>Select</span>
          </li>

          <!-- ko if: selected -->
            <li class='add' data-bind='click: create_invoice'>
              + add invoice
            </li>

            <!-- ko foreach: invoices -->
              <li class='add'>
                <span data-bind='text: identifier'></span>
              </li>
            <!-- /ko -->

          <!-- /ko -->

        <!-- /ko -->
      </ul>

    <!-- /ko -->
  </div>

  <div id='Content'>
    <!-- ko with: selected -->

      <!-- ko if: client -->

        <div id='Client'>
          <h2 data-bind='text: client().data.name'></h2>
          <p>
            <label>Identifier</label>
            <input type='text' data-bind='value: client().data.identifier' />
          </p>
  
          <p>
            <label>Name</label>
            <input type='text' data-bind='value: client().data.name' />
          </p>
  
          <p>
            <label>Email</label>
            <input type='text' data-bind='value: client().data.email' />
          </p>
  
          <p>
            <label>Phone</label>
            <input type='text' data-bind='value: client().data.phone' />
          </p>
  
          <button data-bind='click: client().save'>Save</button>
        </div>

      <!-- /ko -->

      <!-- ko if: invoice -->

        <div id='Invoice'>
          <h2 data-bind='text: invoice().data.identifier'></h2>
          <p>
            <label>Identifier</label>
            <input type='text' data-bind='value: invoice().data.identifier' />
          </p>
  
          <button data-bind='click: identifier().save'>Save</button>
        </div>

      <!-- /ko -->

    <!-- /ko -->
  </div>

</div>

The HTML comments aren't meant to be a replacement, but a supplement. When there is an element available for the control structure, it's wise to use that instead to save on extraneous markup and bytes.