This hasn't been true for a while now, unfortunately. The fact that Internet Explorer is still the most widely used browser out there, which makes it the biggest target, doesn't mean that malicious hackers aren't exploiting security vulnerabilities on other browser platforms.
Today, Google has updated its web browser, Chrome, to version 1.0.154.64, which fixes two serious security vulnerabilities. One of those two lets a remote attacker execute arbitrary code with the privileges of the logged on user - pretty much the worst security problem one can have. Google Chrome has a small market share, and once upon a time you didn't have to worry (much) even about critical security issues like this one, because no one would exploit them. Nowadays, I'm not so sure.
Recently, Mozilla also updated Firefox, fixing several security issues. Just days after version 3.0.9. was released, there was another security and stability update. This may sound like business as usual, but security problems and updates for Firefox are showing up at an increasingly fast rate.
At the same time, malicious hackers have been running rampant on social networks such as Facebook and Twitter with a bunch of phishing attacks and hacks.
As a result, it's once again getting harder and harder to feel safe online. Although most web browsers are now regularly and automatically updated whenever a security issue is confirmed (in Opera this functionality is coming in the next major version, Opera 10), a large number of users never have the latest, most secure version of their web browser.
An interesting new paper on how different web browsers do security updates reveals that the implementation of this feature can have a big impact on the overall security. Google Chrome is leading the pack; it checks for updates every 5 hours, silently updates to the most secure version without the need to restart the browser, and doesn't even let users disable the silent updates. As a result, 97% of users have the latest, most secure version installed.