Uhm... well, here's how it went for me:
1º flash with the new rom (4.0.3) - it booted normally (just a bit slow), some on-line related services went off (I don't have wi-fi here), then any service in memory started shutting off with no appearent reason. Any app would shut itself down very quickly. Re-booted it, and the loading screen with the winking droid guy started showing graphic corruption.
2º flash with the new rom (4.0.3) - No graphic corruption on loading screen, but the services went down so fast I almost even couldn't unlock the screen. The messages were like this: "Unfortunately, the service (any service... from Launcher to Camera to UI (?)) has stopped". I couldn't run a single app.
3º flash with the old rom (2.4.3) - Everything as normal as ever.
4º flash with the new rom (4.0.3) - this time was the most "normal" boot with the new rom... I could play 4 levels of the stock Angry Birds. Still, services going down. I could launch the GBA emulator, and the intro of Castlevania Aria of Sorrow looked glitched. Gameboid stopped working, Android froze.
5º flash with the old rom (2.4.3) - Everything as normal as ever.
6º flash with the new rom (4.0.3) - A mix between my first and fourth attempts. Android froze several times.
Last flash with the old rom (2.4.3) - Everything as normal as ever. I'm currently using this one since I'm in the middle of a Pokemon Fire Red playthrough
So... I could be doing something wrong maybe? If I'm doing things right, and my unit is not faulty... then the ICS rom could be buggy? I hope this helps a bit and someone else shares his/her experience with the new rom.