The symlink task appears to have moved to ant-nodeps in ant 1.7.1, which means that :jar-strategy :symlink is currently broken. Add ant-nodeps as a dependency to get this working again.
This is a personal itch, driven by a desire to use a task incompatible with 1.6.5 [ant-deb-task]. Testing over the course of the day suggests that the problems noted here: http://groups.google.com/group/leiningen/browse_thread/thread/3a0118282d0394b0/a65645fca0f6f73c?lnk=gst&q=ant#a65645fca0f6f don't seem to be happening anymore. A brief attempt to use 1.8.1 ended in tears. Without doing much research, it looks like lancet or leiningen will need to be updated to work with that version.
Will be removed again in 2.0.