At the moment Gzip is used as the default compression method for Debian binary packages, but Ubuntu developers have been interested in switching to XZ. Why XZ? "Fedora is doing it," as explained on the summit session page. But more specifically, XZ offers a much better compression ratio than Gzip and other common compression methods, which would lead to a significant reduction in the archive size. The archive would be smaller, Debian package downloads would be quicker for users on slower Internet connections, and it's pretty much a win across the board. XZ does require slightly more CPU usage for decompression, but nothing really significant for modern hardware. Fedora has been successfully using XZ with RPMs and live images.
Debian developers have also been wanting to switch their package compression method from Gzip to XZ. As I explained in Debian Wheezy To Take Up 73 CDs Or 11 DVDs back during DebConf Nicaragua, "There's a release goal to compress packages using XZ rather than Gzip. Debian developers have found packages to be about 20% smaller when using XZ rather than Gzip. Compressing XZ packages does take significantly longer than Gzip, but when it comes to extracting the archive the time isn't noticeably longer nor is the memory usage up greatly."
We will see if they manage to make the move for Ubuntu 13.04 to XZ. If they do so they will also be back-porting the XZ-capable dpkg package to at least Ubuntu 10.04 LTS "Lucid Lynx" so that Ubuntu OS upgrades will go smoothly.