I personally flashed ICS (non rooted) yesterday (Netherlands). And succesfully returned to GB.
And i DID full wipe my device from system menu.
Maybe i was lucky because i knew i could brick my phone....but i wanted to test what will happen
Steps: (I was on rooted GB)
* Full wiped phone via CWM
* Flashed stock GB via Odin (N7000XENKK1)
* Updated ICS (XXLPY) via KIES.
* After installation i FULL WIPED my device via system menu (factory reset)
* Booted up and played a little with ICS

* Turned phone into download and flashed stock N7000XENKK1 direct via PC Odin.
* I'm still alive!!! (thank you god! hahaha)
* Now i'm back to my beloved DarkyROM 3.4.1 (full rooted)
* And of journey
I Know i was lucky! Bricking my phone could occur! I do not recommendfull wipe while on ICS!! I was only feeding my own curiosity