In iconv/iconv.h the definition of iconv is:
extern size_t iconv (iconv_t __cd, char **__restrict __inbuf,
                     size_t *__restrict __inbytesleft,
                     char **__restrict __outbuf,
                     size_t *__restrict __outbytesleft);

which is wrong.  
The second argument should be "const char **__restrict __inbuf".  This has been
fixed for sometime now in the libiconv project
http://libiconv.cvs.sourceforge.net/*checkout*/libiconv/libiconv/include/iconv.h.in
but not in glibc, which makes it hard to write code which is correct for both.

I checked for this bug against the glibc CVS on 21 Feb 2007