old thread but i thought i would post my answer as it does not require an existing map doc
this does assume that you have arcMap installed and not just arcpy. but who is using arcpy w/o having ArcMap installed?
{
map_file = r'c:\test\test_map..mxd'
# open and close a file
open(map_file,'r').close()
# open the file with default application
os.startfile(map_file)
# sleep for a couple minutes to allow the file to be fully opened
time.sleep(120)
# kill the application in a pretty violent way yet not corrupting the file.
os.system("taskkill /im arcmap.exe /f")
# work with yout map
mxd = arcpy.mapping.MapDocument(map_file)
}