I can't remember exactly why I stopped using sbcl. It used to be my default lisp. I suspect that when I started using slime I just found that cmucl was easier. I'm happy to report, however, that sbcl works with slime (and has done for a while); I was just to busy/lazy to give it a try.
I released the first version of the IFS Designer that does animation and the first feedback that I get is that it doesn't work under sbcl. After a bit of investigation I found that it actually does work but sbcl was emitting a warning every time I referenced an sdl:surface. An IFS typically requires 1000s of points to be drawn so getting a warning for each was a big performance problem. A little help from sbcl-help and some type declarations and all is well. Get the 0.1.1 version, and thanks Zach, I'll be testing under sb and cmu cl from now on.
Under sbcl the IFS Designer is actually faster than under cmucl now. From not working to faster than my development environment, how amusing.