Sorry if this is a dupe, I just joined the list.
It seems I now have to evaluate
(setq slime-lisp-implementations '((sbcl ("/usr/local/bin/sbcl" "--dynamic-space-size 1000") :directory "~/prg")))
to be able to start slime. This is probably non-sensical, but anyway, it works. Is the directory supposed to be the directory of the current buffer, and is slime supposed to find it out by itself?