Xcode4 has a tiny debugger area at the bottom/center of the screen (you can show/hide this area with the view buttons in the toolbar):
I've found that for viewing logs, however, this tiny frame is just not sufficient. Xcode 3 used to pop open a new window for output logs. There's actually a pretty good workaround for this in Xcode 4, however: you can view logs in the big editor window in the center of the screen.
In the top of the Navigator frame, click on the Logs icon . From there you'll see a list of all your recent logs, both build logs and the debugger console. Click on the log name on the left to view it.
If you'd like to show this frame automatically when you build and run your app, go to the Xcode menu -> Behaviors -> Edit Behaviors. Click on the "Build starts" option on the left, and on the right side check the "show" checkbox for the "Log Navigator".
Similarly, you can configure the "Run starts" behavior to navigate to the current log.