Also, in any OS X application with multiple windows you can use Command-` (i.e. Command + backtick key) to switch between windows of the active application.
I'll add this because it has happened to me - sometimes you're in all in one mode and you still don't see your code, just the file name that you selected at the top of the window - this is because there is a horizontal divider that is pulled all the way to the bottom of the window. Look for 1 little dot at the bottom of the window in the center, if you drag that up you'll be able to see your code again.
All-in-One is kinda nice. It seems it should be the default. It still doesn't make the documentation window of any use though. I guess it has a little less chance of being hidden under a pile of other windows now. Command+backtick doesn't seem to do anything on my mac.
Strange that Cmd-backtick doesn't work for you. Try this: Open up System Preferences, select "Keyboard", change to the "Keyboard Shortcuts" tab, and highlight "Keyboard & Text Input" on the left. Is the "Move focus to next window in application" box checked? What's its shortcut?
Also, long-clicking on the dock icon just activates Expose for Application Windows. I have it set-up to activate upon mousing to the top right corner and really like it. You don't have to wait for the delay of clicking and holding on the dock, though you'll still have to scan for the correct window.
I wish I could vote you up more than once. Not using all-in-one layout in XCode is Considered Harmful. This is even more important than binding Open Quickly to CMD-O or some other quick key shortcut.