I'd argue that they have the most incentive for the users to move off of ie5. That being said a message that tells users how to update would be much better then a partially rendered mess.
What if the power goes out? What if your main development PC goes down?
The question is really how often does this happen and how long is it out for. Personally I don't remember the last time my internet was out for more then an hour, and that wasn't even in the last year.
Google Docs must have the same problem, how do they mitigate it?