Skype has just been uploaded to the "partner" official Ubuntu 10.04 Lucid Lynx repository - correct me if I'm wrong but I think this is the first time Skype is uploaded to an official Ubuntu repository (other than Medibuntu obviously - which by the way doesn't include Skype anymore).
So to install Skype in Ubuntu 10.04 Lucid Lynx from the partner repository, go to System > Administration > Software Sources and on the "Other software" tab, enable (check the box next to it) the "http://archive.canonical.com/ubuntu lucid partner" repository.
Then simply do:
sudo apt-get update && sudo apt-get install skype
You can also manually download Skype from Launchpad (but that's just like downloading it from Skype's website so it's advisable to enable the Lucid Partner repository so you'll receive updates).