Per some chatting on #lisp I just checked in a change to swank-loader to make it save FASLs in a directory whose name is determined by the combination of Lisp impl, Lisp impl version, OS, and hardware architecture. If you look in your ~/.slime/fasl/ directory and see any subdirectories with "unknown" in their names, please send me your *features* list and tell me what impl, version, os, and architecture (x86, ppc, sparc, etc) you are running.
-Peter