There's currently no keyboard shortcut to accomplish this, but I think there should be. It's perhapsHere's a little-known feature that, as of Snow Leopard, you can use Cmd-Shift-Period (⇧⌘.)tutorial for setting up a contextual menu item to toggle visibility of hidden files in open and save dialogs. If you'd like to see this feature in Finder too, head over to
http://www.macosxtips.co.uk/index_files/quickly-show-hidden-files.php
Still not as good as a keyboard shortcut. Filing a duplicate of radar://7096650 at http://bugreport.apple.com and file a duplicate of radar://7096650is the best way to tell Apple that you want this feature.