If you'd like the key they used to sign RPMs with, I copied it below. I got it from this Fedora forums posting and also using wayback on http://gd.tuwien.ac.at/infosys/phone/skype/rpm-public-key.asc which I saw mentioned a few places.
The key by itself is worthless if the skype isn't actually signing their RPMs which is indeed the case. The gpg check is complaining because the RPM isn't signed, not because you don't have the key.
If you run
rpm -K some.rpm you can check to see if an RPM is signed. Notice in the output in the command below, the skype RPM does does not list 'gpg' anywhere whereas the signed 'at' RPM from CentOS does and the signed 'aspell' from Scientific Linux does. You can run
rpm -Kv some.rpm if you want more verbose output. The RPM website has more info on signing RPMs and how to tell if they're signed if you're interested.
$ rpm -K skype-18.104.22.168-fc.i586.rpm
skype-22.214.171.124-fc.i586.rpm: sha1 md5 OK
# This is a CentOS RPM that I have already imported the key for.
$ rpm -K at-3.1.10-43.el6.i686.rpm
at-3.1.10-43.el6.i686.rpm: rsa sha1 (md5) pgp md5 OK
# This is a Scientific Linux RPM that I do not have a key for.
# It complains I don't have it but still shows the RPM as being signed.
$ rpm -K aspell-0.60.6-12.el6.i686.rpm
aspell-0.60.6-12.el6.i686.rpm: (SHA1) DSA sha1 md5 (GPG) NOT OK (MISSING KEYS: GPG#192a7d7d)
In summary, the problem you have isn't that you can't get their GPG key, it's that they don't sign their RPMs. You're going to have to go without the gpgcheck to get skype installed.
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.2.4 (GNU/Linux)
-----END PGP PUBLIC KEY BLOCK-----