Now that I think about it, OpenID is as dangerous as it is helpful. Less passwords for you to remember also means less passwords for a scammer to have to guess if you end up falling for some lame phishing scam....of course having 1 password that you memorize is still better than having to write down a bunch of passwords in a little book that has the potential to be stolen. Still...
I think most people, myself included, already use a same password on multiple sites. Not saying it's particularly security-conscious, just that it's already what a lot of people do.
Also, they said you can use multiple openID accounts if you want, so you could still have different password for each site if you so choose.