Compiling the current kernel with gcc 3.2.2 I get the followin error:

../../../../kern/subr_disk.c: In function `diskerr':
../../../../kern/subr_disk.c:151: warning: zero-length format string

The code in question looks like:

                static const char fmt[] = "";
                log(pri, fmt);

Which indeed has azero-length format.  Not sure how it should fixed.  I can, 
of course, bypass the error be turning off the -Werror compiler flag.

Sverre