I had a whole post outlined with all the steps I went through to manually install VirtualBox 3.0 in Ubuntu 9.04 Jaunty, but decided to wait a couple days until 9.10 Karmic came out before finalizing my system setup. As I hoped, it was a piece of cake in Karmic since VirtualBox 3.0 is now listed in the new Ubuntu Software Center!
I just launched the “Ubuntu Software Center” from the Applications menu. When the Software Center opened, I typed “virtualbox” in the search bar in the upper-right corner. “VirtualBox OSE” was listed. (For reference, I later found it under the Accessories category in the Software Center.) I double-clicked on it to get more info, and confirmed that it’s a 3.0+ package (3.0.8 specifically), and clicked the Install button. After entering my password, the Software Center downloaded and installed it without a hitch. Now I can just run Applications -> Accessories -> VirtualBox OSE to launch.
No adding new repositories. No terminal required. It’s almost too easy! Good job, Cannonical.