Experienced this as well. And it's not a change in the security settings on the Exchange server. It's a change in Android behavior. I just don't know if it's ICS-related or CM-related (I haven't set up Exchange on anything but CM-based ICS ROMs). But yeah, on GB I didn't have to enter the PIN every single time I unlocked the phone. Now I do. So far I haven't figured out a way around it (other than using a 3rd party Exchange client, which I hate).
If I do, I'll be sure to let you know.
You might have used security removed "Email.apk" in the GB though or ROM with security removed "Email.apk".