I've had problems with Xcode 4 (4.3.1 - 4E1019).
After a few hours maybe less, my code isn't highlighting correctly, it seems Xcode loses all sense of Cocoa and I end up with the example screenshot, I can't jump to methods (by clicking on a method holding down cmd key) - this should work as expected and it doesn't even work for the Cocoa base classes.
The only way I've found to sort this out is to go to Organizer > Project and delete the projects derived data, then Quit and relaunch Xcode, but it's not long before it goes wrong again.
Has anyone else had this problem ? It's really slowing me down. If anyone has any tips, that would be great.