Under GCC 4.0.2
This code (friend_bug.cpp)
----------------------------------
struct A {
  friend void func() {}
};

int main() {
  func();
}

----------------------------------

Compiles without any diagnostic.

The system is (uname -a):

Linux andre.localdomain 2.6.11-1.1369_FC4 #1 Thu Jun 2 22:53:35 EDT 2005 i586 i586 i386 GNU/Linux

I compiled this code with:
g++ -v -save-temps -Wall -Wextra --pedantic-errors -std=c++98 -xc++ friend_bug.cpp 2>err_output.txt

Output on stderr of g++ is:
-----------------------------
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,java,f95,ada --enable-java-awt=gtk --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --host=i386-redhat-linux
Thread model: posix
gcc version 4.0.2 20051125 (Red Hat 4.0.2-8)
 /usr/libexec/gcc/i386-redhat-linux/4.0.2/cc1plus -E -quiet -v -D_GNU_SOURCE friend_bug.cpp -std=c++98 -Wall -Wextra -pedantic-errors -fpch-preprocess -o friend_bug.ii
ignoring nonexistent directory "/usr/lib/gcc/i386-redhat-linux/4.0.0/include"
ignoring duplicate directory "/usr/local/include"
ignoring nonexistent directory "/usr/lib/gcc/i386-redhat-linux/4.0.2/../../../../i386-redhat-linux/include"
ignoring duplicate directory "/usr/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include
 /usr/local/include
 /usr/include/glib-2.0
 /usr/include/gtk-2.0/
 /usr/include/pango-1.0
 /usr/lib/gtk-2.0/
 /usr/lib/gcc/i386-redhat-linux/4.0.2/../../../../include/c++/4.0.2
 /usr/lib/gcc/i386-redhat-linux/4.0.2/../../../../include/c++/4.0.2/i386-redhat-linux
 /usr/lib/gcc/i386-redhat-linux/4.0.2/../../../../include/c++/4.0.2/backward
 /usr/lib/gcc/i386-redhat-linux/4.0.2/include
End of search list.
 /usr/libexec/gcc/i386-redhat-linux/4.0.2/cc1plus -fpreprocessed friend_bug.ii -quiet -dumpbase friend_bug.cpp -auxbase friend_bug -Wall -Wextra -pedantic-errors -std=c++98 -version -o friend_bug.s
GNU C++ version 4.0.2 20051125 (Red Hat 4.0.2-8) (i386-redhat-linux)
	compiled by GNU C version 4.0.2 20051125 (Red Hat 4.0.2-8).
GGC heuristics: --param ggc-min-expand=42 --param ggc-min-heapsize=23863
 as -V -Qy -o friend_bug.o friend_bug.s
GNU assembler version 2.16.91.0.6 (i386-redhat-linux) using BFD version 2.16.91.0.6 20060212
 /usr/libexec/gcc/i386-redhat-linux/4.0.2/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/gcc/i386-redhat-linux/4.0.2/../../../crt1.o /usr/lib/gcc/i386-redhat-linux/4.0.2/../../../crti.o /usr/lib/gcc/i386-redhat-linux/4.0.2/crtbegin.o -L/usr/lib/gcc/i386-redhat-linux/4.0.2 -L/usr/lib/gcc/i386-redhat-linux/4.0.2 -L/usr/lib/gcc/i386-redhat-linux/4.0.2/../../.. friend_bug.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/i386-redhat-linux/4.0.2/crtend.o /usr/lib/gcc/i386-redhat-linux/4.0.2/../../../crtn.o
-----------------------------

The preprocessed output is:
-----------------------------
# 1 "friend_bug.cpp"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "friend_bug.cpp"
struct A {
  friend void func() {}
};

int main() {
  func();
}
-----------------------------

ACTUAL BEHAVIOR:
Compilation without any diagnostic message.

EXPECTED BEHAVIOR:
A diagnostic message is needed.
This code is ill-formed, since the scope of the friend declaration (which is also a definition) must be limited to the scope of the A structure.
This issue as been discussed on comp.std.c++

And, with -std=c++98 --pedantic-errors I would expect an error.

The comp.std.c++ discussion can be read at:
http://groups.google.com/group/comp.std.c++/browse_thread/thread/3c18fce2a7e57da3