It appears that there are still many OG Transformer TF101 owners out there that have yet to get their Ice Cream Sandwich update, and Asus has noticed this. They kindly went to the trouble of posting on their UK Facebook some detailed instructions on how to get the update manually. They even gently prodded users to go get it, as you can see in the pic above.
It is important to note, that even if you follow these instructions, you will still need to be careful to get the 4.0.3 update that is available for your region, as they are region-specific. Here's a quote from the AndroidPolice article with some final thoughts,
Source: TransformerForums via AndroidPolice and Asus Support Pages and Facebook - Asus UKOnce you figure out which is the correct version for you, simply extract the contents of the downloaded folder, drop it on the root of your TF's internal storage, and the rest should happen automatically.