There has been some noise on #lisp the past days about current usocket using an internal SBCL feature that's breaking with current SBCL. Try downgrading your SBCL to see if that solves the problem, and if it does, wait for an usocket update.
I recently patched cl-postgres to use the non-portable socket interface when on ACL, I think I had to change only three lines. You could try a similar thing for SBCL. Attila also talked about having cl-postgres use iolib (though that has the disadvantage of a huge set of dependencies).
Best, Marijn