In Xcode 3 the run log's name has changed to Console and has become part of the debugger window. The console is initially invisible, which makes locating it difficult. There are two ways to reveal the console. First, you can choose Run > Console to show the console, and you will see the console at the bottom of the debugger window. Second, you can drag the splitter bar at the bottom of the debugger window to reveal the console.
Update (January 16, 2008)
Xcode's debugger window has a splitter bar for the console only if you're using Xcode's All-In-One layout. If you're using the Default or Condensed layouts, the console is a separate window. Clicking the Console button on the debugger window toolbar opens the console for the Default and Condensed layouts.