A new keyboard shortcut in Xcode4 allows you to quickly specify where a file opens in the editor.

Pressing Shift-Option and clicking on the file name in the Project Navigator brings up a quicklook type view in which you can click the location to display the new file. You can select the primary editor pane, the current assistant pane or a new assistant pane. A quick swipe in the 'quickview' window even lets you open the file in a new window. You can also select to open the file in a new tab.

Very handy. Very cool.