My project was created in Xcode 3, I\'m opening it in Xcode 4 and notice the following:
With regard to "Jump to Definition", try clicking on the Scheme selection button (the one near the top right of the window that displays a list of builds, devices, and simulators) then click back in the code editor and try Command-click or "Jump to Definition" again.
I've recently found that whenever "Jump to Definition" stops working I can do this and it works again for awhile. It's not a fix but has been a useful workaround.