Since 1.487, Jenkins apparently attempts to run a graphical JNLP installer on all OS X slaves. This is a bad idea because some OS X slaves (such as Xserve-based ones) will not let the jenkins user connect to a window manager, so starting the slave connection always fails.
A fix suggested by Kenny Ayers on the mailing list is to pass -Djava.awt.headless=true to the JVM, which works. But this should be a more readily accessible option in the slave settings UI - especially considering that this new behavior actively prevents previously working slaves from working.
(I would even argue that this behavior should not be the default, but as long as there's an option, I'm happy.)