OK, so several of us have upgraded to java 7 (which is what Apple has told us all to do), and there are plenty of places where there are instructions for "downgrading" (removing java 7), but all these instructions seem to only actually affect java as a plugin and java webstart. I followed the instructions on the Oracle site. The java "system settings" applet is no longer available, but when I run tekkit, I'm still using java 7.
Downgrading to an admittedly insecure version of java isn't a great solution. Is there somewhere I can download the source code for the launcher? Perhaps I can track the real problem (which I suspect is related to using 1.2.5 with java 7, but it may not be).