In Rachel Davies' Agile Mashup talk at QCON 2008, she noted that many teams have a "polishing" iteration, where no new functionality is released.
My team have recently added this: we find we need a bit of space to step back and look at the application from a "big picture" point-of-view. Sometimes it's useful to look at consistency across the application: particularly from a GUI point-of-view. It's also good to make time for exploratory testing. Finally, we like to make some space to incorporate the feedback we've got from the business during the iteration.
Perhaps we should be doing some of this stuff as we go; but at the moment we're finding a polishing iteration just before release is working very well for us.