A true zero-day exploit involving Flash would be a disaster. It could spread rapidly to many many web sites through SQL injection and other attacks. The only defense would be to disable Flash till there was a fix from Adobe (or surf the web and YouTube inside a virtual machine running Linux).
But even with the current attacks being against the old version of Flash,
it may as well be a zero-day because very few people are actually updated to the current version of Flash.
There is no good update mechanism for Flash in the Windows world. The only hope for a widespread update of Flash would be for Microsoft to include it in the Windows Update.
The best way to make sure you have Flash up to date (and several other programs) is to run the
Secunia Online Scan or the
Secunia Personal Software Inspector. Or the
FileHippo Update Checker, but it's not as comprehensive and isn't aimed at security vulnerable updates.