There is a very complex (maybe too complex), Yahoo! Messenger dedicated chatting client, which can bring voice and webcam functionalities to you. It's called GYachE (or GYachI). It has many functionalities... such as webcam and voice calling (if the drivers from Ubuntu can permit this), mail checker, very customizable chatting options and many, many others (trust me, the're so many).
Program is perfect for experts (they should get how it works fast), but the others, it might take a little to learn how to use it and what it can. The interface is complicated, and I think that they could make it a little more... organized, but for me, it's fine. You can hide offline contacts by clicking where it says "Friends: xxx Online: xx", and selecting "Don't Show Offline Buddies".
The current version is 1.2.2, designed for Karmic, and it's pretty buggy. For installation, you must have an internet connection available because it needs some dependencies from the internet.
If the program becomes unstable, let it like that until it starts responding again, otherwise it has bigger chances to crash.
Download link (Karmic Koala):