How is protocol/client version enforced?

[fivebells]

If a security flaw is found in a blockchain client or protocol and a fast fix is needed, is there a fast way for developers to push a revised client out there?  For that matter, what is the earliest version of the canonical bitcoin client that will work on the current network, and how is that enforced?  I saw that Ixcoin had a mandatory update recently; in what sense was it mandatory?

[theymos]

If a security flaw is found in a blockchain client or protocol and a fast fix is needed, is there a fast way for developers to push a revised client out there?

https://en.bitcoin.it/wiki/Alerts

For that matter, what is the earliest version of the canonical bitcoin client that will work on the current network, and how is that enforced?

I believe version 0.1 will still work as a client.

[Red Emerald]

We should have a network wide drill.  See how many sites handle the error messages.  I bet none.

[Atheros]

That would be exciting.