<< To access mute – select a message and from the drop-down options on the Floaty Bar, tap 'Mute'. >>

http://1.bp.blogspot.com/_4GHuwMk1foo/SfiEjc1x3tI/AAAAAAAAAWM/WhaVWHlHu1w/s400/photo.jpg

Source: http://googlemobile.blogspot.com/2009/04/iterative-webapp-gmail-for-mobile-gets.html