I just installed an update via Zune for my 900 that takes it to firmware 2175.2301.8779.12224.
I'm on Rogers in Canada.
It's only two numbers higher than my last firmware (12222), was billed as a "Nokia Update", with no details...
Resetting the phone worked.
After the reset you will (of course) have to set up the phone from scratch - setting up your Live ID, email, apps, etc...
The phone required no updates (updates survived the reset), and ICS does work now.
ICS issue "known" with two workarounds
I've had my Internet Connection Sharing return the "Connection not shared. There's no cellular data connection to share right now." message with a Rogers Lumia 900 on OS 7.10.8773.98, firmware 2175.2301.8779.12222.
According to a Rogers tech I just got...