Apple's released updates for iPhones, iPod touch models and iPads that closes a nasty PDF-based exploit. That same exploit is what made one-click jailbreaking possible, of course, so upgrade if you'd like a sanctioned, secure model or hold off for future workarounds.
Apple's portable devices automatically open PDFs when they're pointed at in a browser, and some web pages can force the iOS browser to load a PDF that, up until now, could get into your device's internals. It's not quite that simple, though, because there's protection against that PDF exploit available to jailbroken devices.
As the Download Squad blog points out, the update is not the long-awaited fix for iPhone 3G performance (your best real option, until then, is a downgrade). The iPad update is numbered 3.2.2, and fixes the same kind of PDF exploit. Both are free downloads that you can grab through iTunes.