I'm relatively new to iOS development with Xcode4, but when I started using it it seemed like the default setting for the split view was to display the file you were editing in the left pane, and then a related file in the right pane, the .h if you're editing a .m, the .m if you're editing a .h, or the .h if you're editing a nib. (I'm not actually sure quite what the correct terminology is for the view, I'm talking about the view you get when you click the little gas-mask looking button, the middle one of the three above the word "Editor" in the top toolbar.)
Anyway, sporadically this syncing breaks, and the two panes will just show whatever file they happened to have last, without updating when the other pane updates. I don't know what this behavior is called so I don't know how to find the option to change it, or what I might be doing to make it sporadically break. I don't know if this is a bug or if I'm just accidentally hitting an obscure keyboard shortcut.