I upgraded from Kubuntu 12.04 to Kubuntu 13.04 on a Dell Vostro 1510, now when I press the power switch, the session does not ask for a confirmation. Instead the computer switches without a warning.

Did something go wrong during my upgrade or is this the intended behaviour?

How can I get the confirmation prompt?

Open System Settings > Power Management, and check the value of "When power button pressed". To get the behaviour you want, it should be set to "Prompt logout dialog".

The "Prompt log out dialogue" is selected already. And yet it does not prompt.

Do you have anything such as systemd or logind running on your system which may be overriding the behaviour of KDE and shutting the system down anyway?

Nope. I have rsyslogd and ksysguardd running, but not the ones you mentioned here above.

Is there any other power management processes running on your system, such as powersaved or similar?

I am not sure which I should be looking for?

How can I print the processes?

You can use KSysguard to view a list of all processes. The relevant processes in this case are likely running as root.

Here we go, the running processes are:

PID TTY TIME CMD
1 ? 00:00:29 init
2 ? 00:00:00 kthreadd
3 ? 00:01:01 ksoftirqd/0
5 ? 00:00:00 kworker/0:0H
7 ? 00:00:00 kworker/u:0H
8 ? 00:00:00 migration/0
9 ? 00:00:00 rcu_bh
10 ? 00:00:30 rcu_sched
11 ? 00:00:00 watchdog/0
12 ? 00:00:00 watchdog/1
13 ? 00:00:18 ksoftirqd/1
16 ? 00:00:00 kworker/1:0H
17 ? 00:00:00 cpuset
18 ? 00:00:00 khelper
19 ? 00:00:00 kdevtmpfs
20 ? 00:00:00 netns
21 ? 00:00:00 bdi-default
22 ? 00:00:00 kintegrityd
23 ? 00:00:00 kblockd
24 ? 00:00:00 ata_sff
25 ? 00:00:00 khubd
26 ? 00:00:00 md
27 ? 00:00:00 devfreq_wq
30 ? 00:00:00 khungtaskd
31 ? 00:00:06 kswapd0
32 ? 00:00:00 ksmd
33 ? 00:00:00 khugepaged
34 ? 00:00:00 fsnotify_mark
35 ? 00:00:00 ecryptfs-kthrea
36 ? 00:00:00 crypto
47 ? 00:00:00 kthrotld
52 ? 00:00:00 scsi_eh_0
53 ? 00:00:00 scsi_eh_1
55 ? 00:00:00 binder
75 ? 00:00:00 deferwq
76 ? 00:00:00 charger_manager
220 ? 00:00:00 scsi_eh_2
224 ? 00:00:00 scsi_eh_3
225 ? 00:00:00 scsi_eh_4
226 ? 00:00:00 firewire
299 ? 00:00:01 jbd2/sda3-8
300 ? 00:00:00 ext4-dio-unwrit
362 ? 00:00:03 upstart-file-br
391 ? 00:00:03 upstart-udev-br
393 ? 00:00:00 udevd
493 ? 00:00:00 kpsmoused
495 ? 00:00:00 cfg80211
605 ? 00:00:00 wl_event_handle
614 ? 00:00:01 kworker/0:1H
623 ? 00:00:00 hd-audio0
717 ? 00:00:03 upstart-socket-
769 ? 00:00:00 kworker/1:1H
801 ? 00:00:04 jbd2/sda5-8
802 ? 00:00:00 ext4-dio-unwrit
882 ? 00:00:50 modem-manager
884 ? 00:00:00 bluetoothd
912 ? 00:00:00 krfcommd
978 ? 00:00:03 cupsd
983 ? 00:00:07 NetworkManager
985 tty4 00:00:00 getty
988 tty5 00:00:00 getty
994 tty2 00:00:00 getty
995 tty3 00:00:00 getty
1000 tty6 00:00:00 getty
1027 ? 00:00:00 acpid
1032 ? 00:00:00 lightdm
1051 ? 00:00:00 cron
1054 ? 00:00:04 polkitd
1063 tty7 00:37:05 Xorg
1224 tty1 00:00:00 getty
1233 ? 00:00:01 accounts-daemon
1247 ? 00:00:00 kauditd
1249 ? 00:00:02 console-kit-dae
1329 ? 00:00:03 flush-8:0
1395 ? 00:00:03 upowerd
1566 ? 00:00:00 lightdm
1694 ? 00:00:10 udisks-daemon
1695 ? 00:00:00 udisks-daemon
16900 ? 00:00:00 scsi_eh_32
16901 ? 00:00:03 usb-storage
16902 ? 00:00:00 scsi_eh_33
16903 ? 00:00:02 usb-storage
17124 ? 00:00:00 update-apt-xapi
18277 ? 00:00:00 udevd
18278 ? 00:00:00 udevd
22661 ? 00:00:00 migration/1
22703 ? 00:00:00 kworker/u:64
22704 ? 00:00:00 kworker/u:66
22747 ? 00:00:06 kworker/0:3
22976 ? 00:00:00 pppd
23683 ? 00:00:04 kworker/0:2
24653 ? 00:00:02 kworker/1:1
24970 ? 00:00:00 kworker/0:1
25025 ? 00:00:00 kworker/1:2
25067 ? 00:00:00 kworker/0:0
25081 ? 00:00:00 kworker/1:0
25140 ? 00:00:00 cups-browsed

That looks fairly normal. Can you verify that LightDM does not have any options which may be causing it to take actions when the power button is pressed?

Also, it may be wise to check /etc/inittab to make sure no action is configured there as well.
When this logout occurs, is it fairly graceful, or does KDE exit very rapidly, with little warning?

There seems to be no option in the LightGM, related to pressing the power button.

Shutting down from the button is not elegant, the screen turns black and the computer shuts down in text mode and it is very verbose.
Perhaps I should say I find booting very verbose too, I don't remember it was so with 12.04.

Okay, that is definitely indicative that a system level process, rather than KDE is initiating the shutdown procedure.
I've just noticed "acpid" in the output - could you please check it's configuration (or temporarily disable it) to see if it could be the culprit here?

As for why the startup appears very verbose - you may be missing a splash screen. Unfortunately the procedure for enabling/disabling/installing these tends to be distribution specific, so I would recommend asking on your distributions forum regarding that.

I send a STOP signal to acpid after which pressing the power button did not result in shutting down but rather in turning off the screen as per power settings. It seems the acpid is the culprit indeed. Now what can I do to fix this permanently?

I have the splash screen with the 5 logos blurring in, but prior to that it is a all white on black verbose. Shouldn't it be some Kubuntu logo instead?

It should be something graphical involving the Kubuntu logo, yes. Do you use either of the proprietary Nvidia or ATI drivers?

As for removing acpid - you could try either adjusting it's configuration to do nothing, or alternately, uninstall the "acpid" package. You might want to consult the Kubuntu forum before you do that though.