I'm seeing the same problem with the latest cffi from the darcs repository and sbcl 1.0.17.26 [on amd64] It doesn't seem to be a cl-opengl problem, but rather a cffi/sbcl problem, as I get this with any .so as soon as I "defcfun" anything:
caught ERROR: ; (during macroexpansion of (DEFCFUN ("my_function"...) ...)) ; The value NIL ; is not of type ; (OR (VECTOR CHARACTER) (VECTOR NIL) UNSIGNED-BYTE BASE-STRING).
cffi 0.9.2 seems to be working correctly...
serge