I have cl-sdl running with clisp -- well, I only tested sdl-test.lisp and sdl-example1.lisp.
a) my uffi patches (see Sam's pointer, i.e. the sourceforge patches section)
b) remove the ffi:foreign-value in from of the ffi::%slot in get-slot-pointer
-- sigh, I wish UFFI had a specification :-(
c) reload sdl.lisp and sdl-ext.lisp from source after loading the system,
-- this one is a bug in CLISP
d) straight-forward modifications to cl-sdl/ffi/uffi.lisp, as I said.
I used clisp (today's CVS), but 2.33.2 should work if you uncomment the definition in my ffi-patches-cvs.lisp for the foreign-variable constructor (see a).
examples/Mandelbrot.lisp fails with an array out of bounds error at some point, but cmucl does it too.
d) is as follows:
+ add clisp to obtain a nil +null-pointer+
+ turn 3x #+nil to #+clisp to get deref-array/pointer/allocate-foreign-object from the uffi equivalents, no need for a work-around in cl-sdl.