The Twitter systems getting hacked make it much more serious. This kind of hack could happen to any online service provider. Of that, I have zero doubt. But it's more likely to happen to newer vendors with much less mature systems and processes in place. Think Twitter, all of the Twitter-based services (P.S.: never give your password and user name to any of those), as well as all of the new social-network sites that seem to be cropping up every day. Any one of these is a prime candidate for getting hacked because of technical or procedural immaturity.
What can you do to protect yourself? Use a different password for each. Now that may not sound feasible -- and it's probably not. Not without a little help. To manage nearly one hundred passwords, I use 1Password on all of my Mac OS X systems, and I use RoboForm on all of my Windows systems.
These apps make it possible for you to easily manage multiple passwords for all of your Web site logons, as well as automate the filling of registration forms. They'll both also automatically generate strong passwords for you to use.
Which gets us to the One Thing You Need To Do regarding the Twitter hack. Use a different password for e-mail than you do for these social network sites. Having your Twitter account hacked is one thing -- but if that Twitter password is the same as your e-mail password, you're hosed.