Re: '400 different passwords for a variety of different websites'
What does the regularity or location matter for the sites? The irregularly used ones are more important to keep in a PM - there's no chance I'd remember such a password. Take into account the idea of keeping control of your identity online also, ie. registering a username so that others don't, and that gives you more to keep hold of.
Everything is a target for criminals. That's why we constantly hear about hacks of Sony, Yahoo, etc... Its very rare to hear any real danger to a PM provider though.
Biting the hand that feeds IT
