Skype is finally coming in the Official Ubuntu 10.04 Partners Repository. The build process has already been finished and is awaiting approval in the NEW queue.
To install it, you have to enable the partner repository. To do this go to System -> Administration -> Synaptic Package Manager. In the Synaptic Package Manager, go to Edit -> Repositories. In the Software Sources windows, go to the tab Other Software and check http://archive.canonical.com/ubuntu lucid partner and close the window.
Then, open the Terminal and execute:
sudo apt-get update
sudo apt-get install skype