I just tried a recent version of Slime. When I type M-P in the slime-repl buffer I see the previous command I typed, but without any whitespace. For instance, "(cons 1 2)" appears as "(cons12)". In my .emacs file I have slime-repl-history-trim-whitespaces set to t. If I set it to nil, I get the correct history string. It looks like all whitespace is being trimmed, not just leading and trailing spaces.
Emacs 24.3, SBCL 1.1.16.84,
Bob