While working on a site earlier, all icons for the default wp-admin menu items disappeared, though I don't believe it was related to anything I was doing (simply updating a page). This was after updating to 3.1.1, but not immediately after, and I have no plugins that affect the backend of the site in any way. Performed an automatic re-install of 3.1.1, no change. Icons for menu items added by plugins (i.e. cforms) have remained.