Comment 9 for bug 436887

Matthew, those specific dialogs are using "skip-pager-hint" and "skip-taskbar-hint" properties for GtkWindow, so they will never appear in taskbar and/or in Alt+Tab switcher as "[untitled window]".

About the access keys, HIG says to apply them to _any_ button; removing from default button seem to me more a missing feature than an enhancement. Please also consider that:
 1) that's true, you can press Enter key, but if you move focus away from "log out" button (for instance), pressing Enter key you'll activate the Cancel button. In short, Enter key acts on current button, but it isn't a shortcut for a specific button
 2) from GNOME HIG "Provide more than one method to perform keyboard tasks where possible. Users may find some keys and key combinations easier to use than others."[1]

[1] http://library.gnome.org/devel/hig-book/stable/input-keyboard.html.en