enable_trim.sh

#!/bin/bash
#
# Enable TRIM support for 3rd Party SSDs. Works for Mountain Lion, should work on earlier OSes too.
# Tested on 10.8.2, checked for proper operation on 10.8.0, but never booted with the modified kext.
#
# Original source: http://digitaldj.net/2011/07/21/trim-enabler-for-lion/

set -e
set -x

# Back up the file we're patching
sudo cp \
  /System/Library/Extensions/IOAHCIFamily.kext/Contents/PlugIns/IOAHCIBlockStorage.kext/Contents/MacOS/IOAHCIBlockStorage \
  /System/Library/Extensions/IOAHCIFamily.kext/Contents/PlugIns/IOAHCIBlockStorage.kext/Contents/MacOS/IOAHCIBlockStorage.original

# Patch the file to enable TRIM support
# This nulls out the string "APPLE SSD" so that string compares will always pass.
# on 10.8.2, the sequence is Rotational\0APPLE SSD\0MacBook5,1\0
# on 10.8.0, the sequence is Rotational\0\0APPLE SSD\0\0\0Queue Depth\0
# The APPLE SSD is to be replaced with a list of nulls of equal length (9).
sudo perl -pi -e 's|(Rotational\x00{1,20})APPLE SSD(\x00{1,20}[QM])|$1\x00\x00\x00\x00\x00\x00\x00\x00\x00$2|' \
  /System/Library/Extensions/IOAHCIFamily.kext/Contents/PlugIns/IOAHCIBlockStorage.kext/Contents/MacOS/IOAHCIBlockStorage

# Force a reboot of the system's kernel extension cache
sudo touch /System/Library/Extensions/

echo "Now reboot!"

I updated my version to support 10.8.3 and also to add some instructions.