While iOS 3 (iPhone 3.0) introduced Spotlight search for on-device information like contacts, songs, and apps, iOS 4 examples Spotlight to search the Web and Wikipedia as well.
From the main home screen, just swipe to move left to Spotlight, or click the home button to toggle to it, and start typing your search. You may see the aforementioned local results start to pop up but you'll also see a Safari icon saying Search Web and a curiously blank icon saying Search Wikipedia.
Tap those and Spotlight fades, Safari comes up, and you're off and searching the internet.
Already familiar with this? Then let us know if you're using it and how it's working for you. Any advantage to just going straight to Safari or tapping open a Wikipedia app?
For more features, see our complete iOS 4 walkthrough.