Courtesy of Jon Bonilla, Kamailio's Debian APT repository is offering now the packages for version 3.1.0 as well as nightly builds from stable branch 3.1.
So, if you want to get the latest version of branch 3.1, set your apt sources to:
deb http://deb.kamailio.org/kamailio31-nightly lenny main
Supported OSes are Debian Lenny, Squeeze and Ubuntu Lucid.
Check for more details the wiki page: