While the list was sleeping, I adapted Helmut's allegro backend for abcl. It is not quite ready for prime time, but is somewhat usable. Actually, it's already at version 2 (or should I say 0.02): originally, I had to patch swank.lisp, but thanks to Peter Graves, who tracked down pretty printing bugs in abcl, this is no longer necessary. Nevertheless, it's probably too early to include it in the official slime sources, but I thought I'd let you know about it anyway. It's temporarily available here:
http://www.math.bme.hu/~asimon/slime-abcl.tgz
The tar file includes swank-loader.lisp (so back up yours if you untar it in the slime directory) and swank-abcl.lisp. It doesn't work with the latest abcl release, only with the cvs version. (In fact, it's probably better to wait a few hours until anon cvs at sourceforge picks up the latest sources.)
Andras