Bug 1027700

and I updated from F19 with fedup, if it is relevant

This may be relevant. I rebooted the computer during the night. These messages appear in the journal for the user gdm  a few minutes after the reboot and there is nothing after this until the next morning when I login. 

gnome-session[1850]: (gnome-shell:2007): Gjs-WARNING **: JS ERROR: Exception in callback for signal: StatusChanged: TypeError: this._dialog.cancel is not a function
gnome-session[1850]: ScreenShield<._maybeCancelDialog@/usr/share/gnome-shell/js/ui/screenShield.js:640
gnome-session[1850]: wrapper@/usr/share/gjs-1.0/lang.js:213
gnome-session[1850]: ScreenShield<._onStatusChanged@/usr/share/gnome-shell/js/ui/screenShield.js:816
gnome-session[1850]: wrapper@/usr/share/gjs-1.0/lang.js:213
gnome-session[1850]: ScreenShield<._init/<@/usr/share/gnome-shell/js/ui/screenShield.js:537
gnome-session[1850]: _emit@/usr/share/gjs-1.0/signals.js:124
gnome-session[1850]: _convertToNativeSignal@/usr/share/gjs-1.0/overrides/Gio.js:130

I have installed F20 Beta in a virtual machine. I get the same errors.

Seeing the same thing.  Rebooted remotely, came in, monitor was on and sitting at gdm login prompt.

The bug is also happening in GA release. With upgrade and fresh install
https://bugzilla.redhat.com/show_bug.cgi?id=1045934

Really? no response on either report?

1027700
1045934

Did fedup from 19 to 20 on two systems. I am running nVidia drivers using akmod (as opposed to directly installing from nVidia's site).

I see the above non-power-down behavior in GDM in Fed 20. Power-down worked in 19.

The patch in this GNOME bug fixes the problem for me. Hopefully it will be in a gdm update soon.

(In reply to Sergio Pascual from comment #8)
> The patch in this GNOME bug fixes the problem for me. Hopefully it will be
> in a gdm update soon.

Gnome bug fix also works for me

https://git.gnome.org/browse/gnome-shell/commit?id=621e3d0df8abbf9c 

edit /usr/share/gnome-shell/js/gdm/loginDialog.js

add the following 4 lines starting at line 910. Save then restart gdm

+    cancel: function() {
+        this._authPrompt.cancel();
+    },
+

mutter-3.10.3-1.fc20,gnome-shell-3.10.3-1.fc20 has been submitted as an update for Fedora 20.
https://admin.fedoraproject.org/updates/mutter-3.10.3-1.fc20,gnome-shell-3.10.3-1.fc20

Package mutter-3.10.3-1.fc20, gnome-shell-3.10.3-1.fc20:
* should fix your issue,
* was pushed to the Fedora 20 testing repository,
* should be available at your local mirror within two days.
Update it with:
# su -c 'yum update --enablerepo=updates-testing mutter-3.10.3-1.fc20 gnome-shell-3.10.3-1.fc20'
as soon as you are able to.
Please go to the following url:
https://admin.fedoraproject.org/updates/FEDORA-2014-1005/mutter-3.10.3-1.fc20,gnome-shell-3.10.3-1.fc20
then log in and leave karma (feedback).

I confirm that this fix works for me and indeed solves the issue of power save mode.

On top of that, the login screen now shows again which user is already logged in (by a white shade under the name) as it did before (this was gone after fedup to Fedora 20).

Thank you!

mutter-3.10.3-1.fc20, gnome-shell-3.10.3-1.fc20 has been pushed to the Fedora 20 stable repository.  If problems still persist, please make note of it in this bug report.

Oddly enough, after the update I have consistent look-ups on an old machine (a dell GX280)
I tried disabling powersave for gdm (via dconf), but there have not been any improvements. 
If gdm is disabled, everything works nicely; likewise, if a user is logged in on the console (and the screensaver is disabled) the computer does not look up.
This is most puzzling. 

The video card is as follows:
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV370 [Radeon X300]

Incidentally, reverting the change advised by Maurice prevents the problem...