Back in 2011 there was an update to the 360 that bricked consoles. Microsoft sent out replacements to people affected. So it's not unthinkable.
I imagine more people would be affected if the machine is always on, since the updates could be applied even if you're not using the machine and would likely be required in order to connect.
To add an anecdote to this, I had a bricked Xbox 360 due to the famous "red ring of death" within the first two years of its life. Shipped the broken system to Microsoft and received a brand new one in two weeks from Microsoft that has worked ever since. Failing hardware sucks, but the way Microsoft handled it was great.
