Apple just released iOS 4.0.1 for the iPhone 3G, 3GS and 4. The only purpose of the update, according to the release, is that it "Improves the formula to determine how many bars of signal strength to display".
Up to you to decide whether an update that will amount to a cosmetic change is worth the 300MB download. If it is, plug in your iPhone and click Update to grab the latest.
On a more interesting front, Apple also today released iOS 3.2.1 for iPad (is this getting confusing yet?) with improved Wi-Fi connectivity, a copy and paste problem, a video playback glitch and more. Now that definitely sounds like an update worth getting.