Lisp programmers! How could you live without Programmer's Ultimate Security DeskRef? Johan Bockgard found this book, and pointed out the interesting entries on COMPILE ("LISP is an interpreted scripting language. The compile function should be controlled by internal application logic only"), NREVERSE ("This function should never be used to obfuscate data with the goal of protecting it from prying eyes"), and others.
I've recently been moving xach.com and everything it hosts (including Planet Lisp) to a new server running FC4. Almost everything went well, but my first several slime sessions ended with SBCL stalling and chewing up all CPU. After poking around a bit, I found it wasn't just slime; the SBCL threads.impure tests locked up almost immediately.
I spent a few days trying helpful suggestions from Christophe Rhodes, Dan Barlow, and others. Finally, Gábor Melis agreed to log into my system to look at things, and in a few minutes he pointed out that everything worked fine when building with "legacy" gcc 3.2 instead of gcc 4.0. Adding CC=gcc32 to src/runtime/Config.x86-linux and rebuilding produced a stable SBCL.
This is a workaround, not a permanent fix, but to the multitudes of SBCL users who are upgrading to FC4, I hope this saves you all some trouble. Thanks, Gábor!