shift-cmd-opt click on a symbol lets you tell Xcode exactly where in the UI you want the navigated-to file to open.
cmd-ret makes the version editor / assistant (the right hand pane) go away
shift-cmd-Y to hide the debugger
cmd-0 to hide the navigator (left pane)
cmd-[1..n] switch between navigators on left
cmd-opt-[1..n] switch between utilities (thing generally on right)
ctrl-[1..n] switch between various related files in the editor
Overall, Xcode 4 tries to keep things consistent w/the hot keys. Cmd is for mainline commands. Adding modifiers is for analogous commands focused on one particular sub-feature.
(These are just the four that come to mind as the my most pounded upon shortcuts that I'm using constantly! I'm sure it'll change over time as my workflow is refactored into the new hotness.)
