Compiz reached version 0.8.6 a while back - version which brings bug fixes as well as support for KDE 4.4 - but the version in Ubuntu 10.04 Lucid Lynx is still 0.8.4 so if you want to upgrade to Compiz 0.8.6, you can add the following PPA:
sudo add-apt-repository ppa:compiz/ppa && sudo apt-get update sudo apt-get upgrade
If you're using Ubuntu 10.10 Mavrick Meerkat (still in alpha) already, it means you like testing. However, at the time of writing this post, Compiz in Ubuntu 10.10 is still 0.8.4 so you could install the latest Compiz 0.9 beta - the complete re-write of Compiz in C++:
For Compiz 0.9 (Ubuntu 10.10 Maverick Meerkat), the GTK decorator is not working yet!!
sudo add-apt-repository ppa:compiz/ppa && sudo apt-get update sudo apt-get dist-upgrade
Note: I've only tested Compiz 0.8.6 for Lucid.
Update August 24, 2010: The PPAs in this post have been updated. Initially, Compiz 0.9 was in a different PPA but now it's available in the main Compiz PPA - for Ubuntu 10.10 Maverick Meerkat only!
Update November 12, 2010: For a working Compiz 0.9.x PPA (for Ubuntu 10.10 Maverick Meerkat), see: Install Compiz 0.9.2.1 In Ubuntu 10.10 Maverick Meerkat