Open a new tab, show the debugger (cmd-shift-Y) and drag the divider all the way to the top. Close the navigator (cmd-0) and inspector windows (cmd-opt-0), then drag the tab off into a new window.
If you name the tab before you tear it off, e.g., “Debugger”, then set up a Behavior in Xcode’s preferences ( Preferences -> Behaviors -> Run Starts -> Show Tab “Debugger” ), it’ll show that window when you run your app. It’ll even recreate it if you close it.
If you have access to the WWDC 2011 videos, “Maximizing Productivity in Xcode 4” has a lot of great info.
Here’s a screenshot before tearing the tab off:
