It might have taken some time, but Apple just got around to finally pushing out a new version of iOS that fixes the recently revealed exploit that allowed the jailbreakme.com jailbreak.
That's all it really does, so there isn't much to be excited about. It comes in two flavors: 4.0.2 for the iPhone and 3.2.2 for the iPad. Should you install it? Well, it's hard to say. On one hand, it will kill your ability to jailbreak, at least for the time being. Which is unfortunate. On the other hand, it fixes an important security hole that could be exploited by some hackers to do who-knows-what to your iDevice. The decision is yours.