In Xcode 3 I could click a button in the top-right corner of the code editor to switch between header and implementation (.c or .m) files. Now in Xcode 4 there is no button like this around the editor. This would be even more useful now than it was with Xcode 3, because I'd like to click a class in the Symbol view to pull up the relevant .h file, then click a button or keyboard shortcut and get to the .m file.
See this Stack Overflow answer.
"Ctrl+Cmd+Up or Down, but the shortcut seems a bit finicky and sometimes stops working, not yet sure when and why."