Another portable implementation, by Pierre R. Mai, which has been highly bummed for CMUCL with copious input from other developers and users on the cmucl-help mailing list, is available from http://www.pmsf.de/resources/lisp/MD5.html.
and also (adjusted for SBCL compatibility) in SBCL contrib as sb-md5 - just (require :asdf) and (require :sb-md5) to load it.
Alternate download: http://ftp.de.debian.org/debian/pool/main/c/cl-md5/cl-md5_1.8.5.orig.tar.gz (debian to the rescue once again)
(package link stale)
Note that MD5 collisions have been found: Xiaoyun Wang and Dengguo Feng and Xuejia Lai and Hongbo Yu: Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD, so you should consider SHA1 instead.