As suggested above, I filed a support request and got a step by step procedure, written in poor English, but that was nevertheless clear enough.
The first step is to update your SPSS from version 18.0.whatever to 18.0.3. A comprehensive upgrade program to do that is here:
http://support.spss.com/Tech/Products/Statistics/Patches/18.0.3/Downloads.html
I'm downloading that patch right now, but (obviously) I haven't installed it yet.
The second step is to replace a library file with an updated version. The library file is a tiny 48K file buried away in the SPSS program package. AFTER you update to 18.0.3, you burrow down into that package, change the original name from libspssjvm.dylib to libspssjvm.dylib.orig (which inactivates it). Then you copy the new file named libspssjvm.dylib, close everything up, and supposedly you're ready to rock and roll.
That little file was attached to the support ticket I got from SPSS, as described in other messages in this thread (thanks, everybody!). I don't feel comfy sending that out, but it took less than 24 hours to get it officially from SPSS. All I needed to set up an account was the serial number of my site-licensed copy of SPSS.
I'll report back in a few about progress; the SPSS update installer is recommending that I quit all programs now.