Some time back, I posted about how to do this. Well by the time we got to RTM, this had changed.
There isn't a "create placeholder" option when you right-click a cell any more.
When you drag a field into a cell, it creates a placeholder that contains the field. If you right-click the placeholder that was added, you'll see an option to edit the Placeholder Properties. What threw me for a while was that if you right-click the cell, you won't see this option. Turns out you have to right-click the field-name within the cell, not the blank area beside the name within the cell. Thanks to my colleague Jessica Moss for helping me find it.
This really isn't very good UI work in the report designer. It's quite counter-intuitive and different to how cell selection, etc. works in other products like Excel.