Biggest hurdle is not the OS. Problem is that if you don't have a backup of all your database information for the 'i' apps - Mail, iPhoto and so on, then they will not work with an older OS version. When you update the OS those get updated to new configuration as well. Most of the changes are in the user's Library. There is no tool to take them backwards.
The only protection from this is a backup. With a backup from before the update you can just copy everything back over the top. Without a backup you are starting from scratch, just as if you lost that boot drive.
Rick
molṑn labe'
"I am a mortal enemy to arbitrary government and unlimited power. I am naturally very jealous for the rights and liberties of my country, and the least encroachment of those invaluable privileges is apt to make my blood boil."
--Ben Franklin