Thanks Antonio. My video card is indeed a nVidia, that my explain why I don't see the logo.

I still have the problem with KDE and ** any other package I try to install**, including non repo apps such as Skype or Flash Player. This happens both using the GUI and CLI.
See the following two attempts (I removed useless parts):

[root@squarz yum.repos.d]# yum install "K Desktop Environment"
Loaded plugins: refresh-packagekit
Setting up Install Process
No Match for argument: K Desktop Environment
No package K Desktop Environment available.
Nothing to do
[root@squarz yum.repos.d]#

-------

[root@squarz yum.repos.d]# yum groupinstall "KDE (K Desktop Environment)"
Loaded plugins: refresh-packagekit
Setting up Group Process

[...]

Transaction Summary
================================================================================
Install    118 Package(s)
Update       6 Package(s)
Remove       0 Package(s)

Total size: 306 M
Is this ok [y/N]: y
Downloading Packages:

Package kdelibs-common-4.2.2-12.fc11.x86_64.rpm is not signed
[root@squarz yum.repos.d]#

Most probably I do something wrong, but I'd really like to know what!

Thanks for your time,

Enrico Accenti