Summary and Info:

Pacman should improve its output after a kernel update has been installed but the system has not been rebooted yet.

My personal story:

This morning, I ran pacman -Syu and I didn't notice that the kernel was updated & the terminal with the output was closed.
Many hours later - in the afternoon - I was unable to connect to the wifi and had weird errors showing up.
After debugging this issue with the dear folks at #archlinux IRC channel for quite some time, it was suggested to check which kernel I am on.

When running uname -r, I saw the old kernel running, so it was suggested by #archlinux to upgrade kernel.
But running pacman -Syu did not show any new packages, and no kernel updates. Then attention shifted to debugging the package mirrors (pacman.d/mirrorlist), etc.

After some time, I was asked to check pacman -Q linux and it showed the proper kernel version. Then I was told that I need to reboot my machine, and - magically - after a reboot the wifi issue was fixed.

During this process, I ran pacman -Syu many times, and it always had output like this:

---------------8<----------------
:: Synchronizing package databases...
core is up to date
extra is up to date
community is up to date
multilib is up to date
:: Starting full system upgrade...
there is nothing to do
--------------->8----------------

This was shown even though `pacman -Q linux` and `uname -r` returned two different kernel versions.

AFAIK kernel updates are always a bit tricky in terms of drivers, so I think many users are potentielly affected by not rebooting after updating the kernel, and later reporting some problems over the #archlinux IRC channel or other support channels.

I suggest that pacman -Syu shows a notification message such as "WARNING: Your kernel was updated but you did not reboot yet, if you experience any issues please reboot first!" in order to spare other people the same waste of time that I had.