Seems a problem with the system libs (I use MacOS 10.7.5), I just create a file with 6227020800 'a' chars on a Linux env, and the result is:
  8adbd18519be193db41dd5341a260963
I'll try to confirm this.

root@tras2:/var/tmp# pypy create_input.py 
root@tras2:/var/tmp# wc -c md5_test 
6227020800 md5_test
root@tras2:/var/tmp# md5sum md5_test
8adbd18519be193db41dd5341a260963  md5_test
root@tras2:/var/tmp# cat create_input.py 
f = open('md5_test', 'w')
for count in xrange(0, 622702080):
	f.write('aaaaaaaaaa')
f.close()
root@tras2:/var/tmp# cat /proc/version 
Linux version 3.2.13-xxxx-std-ipv6-64 (root@kernel-64.ovh.net) (gcc version 4.3.2 (Debian 4.3.2-1.1) ) #1 SMP Wed Mar 28 11:20:17 UTC 2012