Remember this? It was no joke. Ice Cream Sandwich is indeed hitting the Asus Transformer Prime today, and we've got the OTA file to prove it.
Before you frantically skim the post for the download link, listen up: this will update will break root. So beforehand you are going to want to run OTA RootKeeper, so you don't lose root access. Then you are free to update.
After saving your root access, download this (for U.S. Prime models), and enjoy that ice creamy goodness.
Edit by Artem: I've requested detailed installation instructions from our tipster - we'll update the post when we figure everything out.
Update: Unfortunately, I wasn't able to get reliable instructions, so for the time being note down how to retain root and wait for the upgrade to hit your device OTA.