Until release, 1.6.0 represents the SVN trunk. You can get the source tree via:
svn co https://openser.svn.sourceforge.net/svnroot/openser/trunk kamailio
Another option is to use the debian repository available on http://devel.kamailio.org, it contains daily updated debian packages of the latest trunk version.
By April 2009 all Kamailio and SER modules could be compiled on the new integrated core of SIP Router project, delivering first testing version of SIP Router. As an important milestone has been reached when SIP Router has become ready for testing, further development of Kamailio project has been moved on the GIT repository of SIP Router project at http://sip-router.org.
Important note: SVN trunk is no longer updated.
Check what is new in the Kamailio 1.6.0 version at:
http://sip-router.org/wiki/migration/new-features-for-k-users