ICS is supoose to be a stock android experience. Google has said that ICS is to end fragmentation across the various handsets. Differing UI such as touchwiz, blur and Sense do nothing but slow down updates to people's phones. I'd be shocked if Google allows its partners to slap a new UI on top of ICS.
Nexus phones are designed to run stock Android, but I've read nothing specific about ICS itself being mandated to be stock and only stock. The only specifics Google has offered as to the fragmentation issue is an agreement with manufacturers/carriers to support new devices with updates for the first 18 months which, honestly, doesn't accomplish all that much. Given Android's update history, that'll generally only guarantee one OS version upgrade, two at the most if the phone is released at the right time to span two OS releases.
As for disallowing custom UI's, again, I've read nothing suggesting Google plans to restrict this. As it's one of the primary ways each manufacturer differentiates their product, I'd be shocked if Google can, or has any interest in, doing anything about it beside the aforementioned pledge of 18 months of support.