Got latest ICS update and disabled Tegra Zone, but still had random reboots. So, following some people's swearing that factory reset is the fix, did that last night, wiped everything clean, no restores, reinstalled minimum set of apps clean, disabled tegra zone again, etc etc. Tablet was squeaky clean as of last night... 12 hours later rebooted again during sleep.
Its probably obvious, but there is an uptime counter in settings/about menu, so you can easily see if your tablet rebooted recently. I'll be watching it, but its already clear that nothing fixes it for me.
ASUS people, if you read this, you suck!!!!