Change the transparency levels on all layers inside of your current ArcMap MXD document in one fell swoop!

set_global_layer_transparency.py

##################################################################################################################
#
#	Requirements: You'll need ArcGIS Desktop 10.1 or higher with Python 2.7+ 
#
#	If you want to change Transparency for all the layers in your ArcMap MXD quickly, this will do the trick.
#
#   1. Copy / Paste the initial commands and function definitions below into an MXD's Python command shell, one-by-one
#   2. Use the command "setLayerTransparenciesTo(75)" to change every transparency to 75, or whatever you want
#   	(As a side note, notice that the Group Layers are set to 0 transparency. Play with this if you want!)
#
#	Feel free to use, rewrite, and distribute as you wish.
#
##################################################################################################################

import arcpy

# If you plan on using this code outside of an ArcMap session, swap out "CURRENT" with your MXD path here
mxd = arcpy.mapping.MapDocument("CURRENT")

# The UpdateLayer function lower down requires you provide it an explicit Data Frame. Change this if you need to
df = arcpy.mapping.ListDataFrames( mxd , "Layers" )[0]

# Create a list of all the Layers in your MXD's Data Frame of choice
allLayers = arcpy.mapping.ListLayers( mxd, "*", df )

# This is the function that takes your Transparency input and actually applies it to all the layers you told it to
def setLayerTransparenciesTo ( tpv ):
	for layer in allLayers:
		if layer.isGroupLayer:
			layer.transparency = 0
			layer.visible = True
		else:
			layer.transparency = tpv
			layer.visible = True
		arcpy.mapping.UpdateLayer( df, layer, layer, False )

# This is just an extra function I included so you can confirm that all the layers got changed how you want them to
# You could fancy this simple report to your liking using additional Layer object properties		
def layerReport ():
	for layer in allLayers:
		print layer.isGroupLayer, layer.transparency, layer.name