Apply by doing:
       cd /usr/src
       patch -p0 < 011_sudo.patch

And then rebuild and install sudo:
       cd usr.bin/sudo
       make depend
       make
       make install

Index: usr.bin/sudo/parse.c
===================================================================
RCS file: /cvs/src/usr.bin/sudo/parse.c,v
retrieving revision 1.18
diff -u -r1.18 parse.c
--- usr.bin/sudo/parse.c        4 Dec 2007 15:27:06 -0000       1.18
+++ usr.bin/sudo/parse.c        22 Feb 2009 21:35:30 -0000
@@ -606,9 +606,11 @@
    /*
     * If the user has a supplementary group vector, check it first.
     */
-    for (i = 0; i < user_ngroups; i++) {
-       if (grp->gr_gid == user_groups[i])
-           return(TRUE);
+    if (strcmp(user, user_name) == 0) {
+       for (i = 0; i < user_ngroups; i++) {
+           if (grp->gr_gid == user_groups[i])
+               return(TRUE);
+       }
    }
    if (grp->gr_mem != NULL) {
       for (cur = grp->gr_mem; *cur; cur++) {