TBNL 0.3.12 two days ago (announcement). It now has a special variable *HTTP-ERROR-HANDLER* (defaults to NIL) which can hold a function designator to an error handler for HTTP errors like the 404 Not Found error.
In the following example a simple error handler just handles the 404 error. Every other error returns NIL and TBNL takes over. The function could emit some explanation about the site's structure or just show the wrong part of the URL and make fun of the visitor. :-)
TBNL has fileupload now (since 0.3.1) and there aren't (m)any missing features left. Current versions of mod_lisp must be optained directly from the subversion repository. SBCL now has unicode support. But be careful and test your code!