My God! Why use Linux at all?? Who would you go to if a Linux patches breaks something??

Depends on if you, as a company, offer support for you open-source endeavors. I do (see http://www.linuxuk.org).

I agree that that woudl be awkward if an update just happened to existing tablets, but if the user had to make a manual change (e.g. a repository change) before receiving the update, then it could be made clear at the time that this was no longer a Nokia update.

Says whom, exactly? You? It's already under discussion. The setup required to shield Nokia from liability is being figured out. Now, this doesn't mean they're going to open source everything Diablo, but it does mean the community will be able to ship at least one opt-in update (more, if somebody is interested in maintaining it beyond that).