We're faced with that scenario more times than we like. 
Depending on who you ask, it's "We need to remove old versions to reduce unnecessary issues" on the IT side, and "Touch my CS5.5 and I will beat you" on the user side. LOL
Our position is to remove old versions, so we're testing the Adobe cleaner tool, hoping to be able to script removal of all previous versions. Of course the litmus test is to get CS4 installed on a test Mac for testing.
http://www.adobe.com/support/contact/cscleanertool.html
Man if there's anything this exercise has done is to remind us how much progress Jody/Karl have made in the last few years. Installing CS4 (pre-AAMEE days) is like having toothpicks shoved into your retina. It's sooo much better today.
Don